To write an exercise with subexercises, start the exercise with exer or exera, and then introduce each subexercise with subexer or subexera. You can mix answered and nonanswered subexercises as you wish.
If the exercise begins with a subexercise without any preceding text, it should begin ``exerimmsubexer'' (either may be followed by an ``a''). The remaining subexercises are labeled subexer or subexera as before. The answer to a subexercise beginning with immsubexer should be labeled immsubanswer. The use of immsubexer(a) and immsubanswer keeps the spacing straight (although the current version, January 31, 2023, still doesn't get the spacing right when the exercise has a double-digit number).
I apologize for the necessity of using immsubexer and immsubanswer, but it seems quite complicated to create a version that knows whether the exercise has any beginning text or not.