Does difficult terrain on the material plane impart the same on the ethereal plane?

Question

We encountered a forest which was difficult terrain. Using the Blink spell a character appeared in the ethereal plane and moved some distance.

My warlock with poor stealth spotted a group of centaurs deep in the forest. Rather than getting too close, he moved behind a tree for total cover.

Curious about the centaurs, he wanted to get closer without being seen. He cast blink; on his next turn he readied movement. At the end of his turn the warlock appeared in the ethereal plane and used his readied movement to approach the centaurs to see what they were up to, without being seen.

We didn’t know if the warlock’s movement should be affected by the thick forest underbrush when in the ethereal plane.

Is a character's movement in the ethereal plane hampered by naturally difficult terrain from a forest?

Answer 1

No, it doesn't.

There is nothing in the Dungeon Master's Guide regarding difficult terrain conditions on the Ethereal Plane (either Border or Deep). While it is stated that (page 48):

every location on those planes has a corresponding location on the Ethereal Plane,

this does not mean that the features on the Prime Material plane are copied/replicated, it just means that every point in the 3D space of the Prime is accessible from the Ethereal. If we combine this information with the following:

solid objects on the overlapped plane don't hamper the movement of a creature in the Border Ethereal ... a creature there can move up and down as easily as walking,

we can easily conclude that the movement in the Ethereal is independent of the Prime, and hence not subject to difficult terrain conditions of the Prime.

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Support from the D&D lore

I can add that this description of the Ethereal Plane has been in the D&D lore since a very long time. The 2e A Guide to the Ethereal Plane states (page 12):

A basher on the Ethereal Plane can also pass through solid objects on an adjoining plane without ill effects. In fact, an ethereal traveller can phase through solid stone, ice, magma, and even magical fire, ..., provided the barrier is on the adjoining plane.