Can it be translated as "the summer nights are short"?
You would expect a noun compound for that sentence "Yaz geceleri":)
I answered " The summer nights are short" and it marked it wrong. In english it is the same meaning as "the nights are short in the summer"
Would 'Yazın geceleri kısadır.' (lit.: The nights of summer are short.) work? If 'yaz' is in genitive then why does 'geceler' not have an 'i'? Is 'Yazın geceler' some kind of expression?
"Yaz" is an irregular word. It isn't in the genitive here, but rather just takes that same suffix when used in time expressions.
Why is this a time expression?
So the trick here is that "Yazin" is actually "Yazda" but irregular.
could I say 'yazda geceler kisadir' ?
Nope, "yaz" is an irregular word. You would just have to say "yazın" :)
And as a reminder to myself as much as anybody else, the same goes for "kış" and "kışın," but not "ilkbahar(da)" nor "sonbahar(da).
Geceler - nights
Geceleri- the nights?