With this one, probably more likely middle voice (μέση φωνή).
Passive and middle forms are the same in many cases in Ancient Greek, though, and the two have merged in Modern Greek.
Passive would be for things that are done to you, middle for things you do to yourself.
Incidentally, as far as I know, there are only four verbs (or at least only four common ones) that conjugate like κοιμάμαι:
- κοιμάμαι "sleep"
- θυμάμαι "remember"
- λυπάμαι "regret, be sorry"
- φοβάμαι "fear, be afraid of"
I used " the girl does not sleep" in an overall strengthening exercise, and it was marked incorrect. I realise that in English there is a difference in meaning, but is there a difference in Greek? Previously i have been able to use "does ..." and "is ..." interchangeably.