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 1st-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 1st-level spell with the Aberrant Dragonmark feat.
Best Answer
Reading the whole section it should be pretty clear.
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:
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.