Must I expend a Hit Die to cast a spell using the Aberrant Dragonmark feat?

Question

In a new Eberron game, my DM suggested that I consider taking the Aberrant Dragonmark feat. It suits me well, as I initially wanted to take Magic Initiate.

However, one word in the description is bugging me. It says that when I cast the 1^st-level spell, I "can" expend a Hit Die (emphasis mine):

When you cast the 1st-level spell through your mark, you can expend one of your Hit Dice and roll it.

Must I expend one Hit Die if I'm able to?
What happens if I don't want to?
What happens if I don't have any Hit Die left?

Usually the wording says "you may" and provides further wording if I do or if I don't (or both), but here there's no such stances. So I'm at a loss on how to properly understand the requirement of casting a 1^st-level spell with the Aberrant Dragonmark feat.

Answer 1

Reading the whole section it should be pretty clear.

• [...] You learn that spell and can cast it through your mark. Once you cast it, you must finish a short or long rest before you can cast it again through the mark [...]
• When you cast the 1st-level spell through your mark, you can expend one of your Hit Dice and roll it. If you roll an even number, you gain a number of temporary hit points equal to the number rolled. If you roll an odd number, one random creature within 30 feet of you (not including you) takes force damage equal to the number rolled. If no other creatures are in range, you take the damage.

So the usage of a hit die is completely optional to incur an additional effect as you cast your spell.

To clarify, you can cast your 1st level spell once, with no cost.
To do so again you need to rest – a short rest is enough, while the Magic initiate feat requires a long rest.
As you do, you can spend a Hit Dice for additional effect (temp hit points or damage).

I realize that this is mostly a question about the usage of "can", rather than "may". But there are several other descriptions that use can for optional actions. Like Witch Bolt:

..and on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically

In summary:

Must I expend one Hit Dice if I'm able to?

No.

What happens if I don't want to, or have any Hit Die left?

Your spell is cast in a normal fashion.