Can I still command/control Prime-cast concentration spells while I am in the Border Ethereal?

Question

If I have cast something like Animated Objects or Bigby's Hand and then follow up the next round with casting Etherealness, can I still command/control those spells?

Answer 1

Yes for Bigby's Hand; No for Animate Objects and most other spells

In general, nothing stops you from maintaining concentration and continuing to issue commands to the entities created by such spells even from another plane of existence. However, it seems that the vast majority of spells that allow you to issue commands to something require you to be within a certain range in order to issue commands. This is a problem, because as far as I can find, nothing in the official materials defines distances between things on different planes. Jeremy Crawford interprets this lack of a definition as meaning that if two objects are not on the same plane, they are not within any distance of each other. Effectively, the distance between them is infinite. In Crawford's words:

If two people are on different planes of existence, they are infinitely far away from each other. For example, if I'm on the Material Plane and you're on the Ethereal Plane, we're not within 30 feet of each other.

So, Animate Objects says:

you can mentally command any creature you made with this spell if the creature is within 500 feet of you

As long as you are on the Ethereal Plane, you are not considered within this range and are therefore unable to issue commands to the objects you have animated. In contrast, Bigby's Hand has no such limitation. Here are the relevant excerpts from the spell text:

The hand lasts for the spell's duration, and it moves at your command, mimicking the movements of your own hand.

[...]

When you cast the spell and as a bonus action on your subsequent turns, you can move the hand up to 60 feet and then cause one of the following effects with it.

There is no mention of any range limitation. As long as you can move your own hand, you can control Bigby's Hand from anywhere on any plane.

Unfortunately, it seems that most spells have limitations that prevent you from issuing commands across planar boundaries, although the nature of the limitation varies from spell to spell. For example, Animate Dead and Create Undead both have a range limitation like that of Animate Objects. Mage Hand "vanishes if it is ever more than 30 feet away from you". Unseen Servant disappears if you command it to perform a task more than 60 feet from you, which means you can issue commands, but any command you issue from another plane will cause it to vanish. Dominate Person has no range limitation, but explicitly limits the telepathic link to the same plane. A zombie created by Finger of Death only obeys commands that you speak verbally, which makes it impossible to issue commands from another plane, unless you have a way to transmit your voice across planes.

In short, you'll need to read your spells carefully, because each one that creates or takes control of an entity has different requirements you must fulfill in order to exert that control, and many of those requirements preclude cross-planar control.

However, your DM could rule otherwise

Lastly, I will mention that as of the 2019 Sage Advice Compendium, Jeremy Crawford's tweets are no longer official rulings. I can't find any official rule that specifically describes how distance works (or doesn't work) between the Material and Border Ethereal Planes, which means that the rules as written leave this determination entirely up to the DM. Crawford's tweet interprets the lack of any definition for inter-planar distance as meaning that things on different planes are never within any distance of each other, but your DM could just as easily decide to fill in the blanks in the rules by defining a distance measure between the Material and Border Ethereal Planes.

Addendum: Indirect evidence of inter-planar distance measure

I only found this much later, and I'm not sure how to integrate it into the rest of the answer, but it turns out while distance between planes is never defined, there are a few instances where official materials assume that it is defined. The description for the Wand of Enemy Detection says (emphasis added):

For the next minute, you know the direction of the nearest creature hostile to you within 60 feet, but not its distance from you. The wand can sense the presence of hostile creatures that are ethereal, invisible, disguised, or hidden, as well as those in plain sight.

If the distance between creatures on different planes was infinite or undefined, then it wouldn't be possible for a creature to be both within 60 feet and ethereal at the same time (assuming the wand's user is on the material plane), so this text would make no sense unless distances between the material and ethereal were defined. Similarly, the spell Mordenkainen’s Faithful Hound says (emphasis added):

When a Small or larger creature comes within 30 feet of it without first speaking the password that you specify when you cast this spell, the hound starts barking loudly. The hound sees invisible creatures and can see into the Ethereal Plane. It ignores illusions.

Once again, the ability to see into the Ethereal Plane would be meaningless unless a distance measure was defined. Unfortunately, these examples still don't supply such a definition, so the DM will need to make a ruling if they want it to be defined.

Answer 2

Yes.

PHB p. 203 states that the only things that can break your concentration on said spell are when one of the following occurs:

1. You take damage and fail a Constitution saving throw.
2. You cast another spell that requires concentration.
3. You are killed or incapacitated.

The spell in question, Etherealness, states that you cannot interact with creatures or objects that are not on the same plane as you unless they have a special ability or magic allows them to. In this case, since you still have a magical connection to them, you still control and command them.

Answer 3

Yes, because the Border Ethereal is Special

Unlike any other two planes, the Border Ethereal specifically overlaps with any Inner Plane that it touches - the two are contiguous and any point on one is a measurable distance to a point on the other. This is shown by the descriptions of at least one item and one spell, as Ryan C. Thompson's answer shows, but also in the description of the Border Ethereal itself.

Observers on the Prime don't see the Border Ethereal as adding any distance to there.

The Wand of Enemy Detection says:

For the next minute, you know the direction of the nearest creature hostile to you within 60 feet, but not its distance from you. The wand can sense the presence of hostile creatures that are ethereal, invisible, disguised, or hidden, as well as those in plain sight.

If the distance between creatures on different planes was infinite or undefined, then it wouldn't be possible for a creature to be both within 60 feet and ethereal at the same time (assuming the wand's user is on a prime material plane), so this text would make no sense unless distances between the material and ethereal were defined.

Similarly, the spell Mordenkainen’s Faithful Hound says:

When a Small or larger creature comes within 30 feet of [the hound] without first speaking the password that you specify when you cast this spell, the hound starts barking loudly. The hound sees invisible creatures and can see into the Ethereal Plane. It ignores illusions.

Once again, the ability to see into the Ethereal Plane would be meaningless unless a distance measure was defined. Unfortunately, these examples still don't supply such a definition of distance. That is, this magic item and spell tell us that something on the Border Ethereal is within 30 feet of us on the Prime, but don't say how distances relate between the two planes. If something is 20 feet away from me on the Border Ethereal and I walk five feet away from it on the Prime, how far away from it am I now? If it moves five feet closer to me on the Border Ethereal, how much ground did it close on the Prime? The wand and the hound don't tell us that.

Fortunately, the DMG description of the Ethereal plane does:

The Ethereal Plane is a misty, fog-bound dimension. Its "shores," called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane.

According to the DMG description, while part of the Ethereal Plane does not overlap the Inner Planes (previous editions called this the 'Deep Ethereal'), part of the Ethereal Plane does overlap ("its shores", called the Border Ethereal). Importantly, where they overlap, there is a one-to-one correspondence of locations. If relative distances did not match between them, there would have to be locations on one that did not exist on the other. Since every location on one matches a location on the other, they must have equal distances as well. Thus, the distance between two locations on an Inner Plane is the same as the distance between the same two corresponding locations on the Border Ethereal. Since the DMG makes this point explicitly without mentioning a distance between the planes, and going with the common English meaning of 'overlap', we can conclude that the distance from any Inner Plane location to its corresponding Border Ethereal location is zero. (For a description of this using more formal mathematical vocabulary, see The Material-Ethereal Isometry

Further, the DMG also tells us that

Visibility in the Border Ethereal is limited to 60 feet

and

From the Border Ethereal, a traveler can see into whatever plane it overlaps, but that plane appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy. Ethereal denizens watch the plane as though peering through distorted and frosted glass, and can't see anything beyond 30 feet into the other plane.

Note then that a creature on the Border Ethereal can see 30 feet of the 'overlapped' Prime, but fully 60 feet of the Border Ethereal itself - and no conversion between the two is given. Thus we can assume that the thirty feet of Prime which can be seen corresponds to thirty feet of Border Ethereal, and then the creature can see an additional thirty feet beyond that on the Ethereal only. If the distances on the two planes were not identical, we would need to be told some sort of conversion to understand how much of either could actually be seen. An observer on the Border Ethereal sees distances on the Prime as matching one-to-one.

To answer the question then, since no range is given on Bigby's Hand, you can continue to command that while you are Ethereal regardless of its distance from you (although it gets complicated if you can no longer see the hand or its targets). For Animate Objects, however, "within 500 feet of you" while you are Ethereal and the objects are not translates as "within 500 feet of the location on the Inner Plane corresponding to your current location on the Border Ethereal".

Answer 4

Yes for the hand, but you cannot affect creatures

I find the detailed explanation by Kirt why distances to points in the prime material are the same as if you were on the prime material while you are in the border etheral very convincing.

The distance to the border etheral is officially infinite

However, there is an question and answer in the offical Sage Advise consortium that states:

Can a School of Divination wizard on the Ethereal Plane use Portent on a creature that the wizard can see on the Material Plane? Yes. Portent requires you to be able to see the creature, but it has no range restriction.

The mentioning that portent has no range restriction implies that any such range restriction would make it impossible to use the spell, which would indicate that the distance to the prime material is infinite or at least undefined.

How does this match with the Wand and Hound? Could distance be assymetric in this case, so it is defined from the material plane, but not from the Ethreal Plane?

It might not matter

However this may be, Etherealness' text says:

While on the Ethereal Plane, you can only affect and be affected by other creatures on that plane. Creatures that aren't on the Ethereal Plane can't perceive you and can't interact with you, unless a special ability or magic has given them the ability to do so.

So even if you could control Bigby's Hand or objects you animated with Animate Objects, the typical purpose of doing so is to attack creatures. And Etherealness explicitly states that you can only affect creatures on the Ethereal Plane. So you could not affect them, neither through the use of these spells nor otherwise.

The exception that creatures that are not on the Ethereal Plane can interact with you if they have a special ability or magic does not necessarily mean that you can affect them. Their ability to do so may well be assymetric. Spells typically only do what they say they do, and nothing in the spell says you can interact with them.

At least there is enough uncertainty here for a DM to make a ruling as they like.

You will be able to affect objects with those spells that are not on the Ethereal Plane, as those are not excluded by the spell (although I would argue that you cannot affect objects with the hand, either, this is not the consensus opinon).

Conclusion

• Animate Objects won't work, as the distance is infinite. Even if you rule equidistance, it cannot affect creatures
• Bigby's Hand won't work to affect creatures (your DM decides if it affects objects)