Translation:I opened the window because it was hot.
To add on to Jennifer's question: is the use of γιατί to mean "because" considered perfectly good, acceptable Greek, or is it something one would only say in very casual conversation?
I ask because in Modern Hebrew, some speakers also use the word למה (láma) which means "why", instead of one of the words meaning "because". However, this is not considered correct standard Hebrew at all. (It sounds very bad to my ears, ignorant and illiterate at best and completely non-native at worst, although in recent years I've only heard it used deliberately and ironically.)
Γιατί is used as both why and because.
Επειδή only means because.
is the use of γιατί to mean "because" considered perfectly good, acceptable Greek, or is it something one would only say in very casual conversation?
It's perfectly fine to use γιατί in any context, you will lose no points for informal use! :) Having said that, επειδή carries a tiny bit more weight to it, probably because it's more specific.
Ζέστη is a noun and means "warmth, heat" so ήταν ζέστη would mean "it was heat" which is wrong clearly. In english, hot is an adjective, so you have to say "it was hot" with it implying the weather. In Greek, you can say ο καιρός ήταν ζεστός=the weather was hot, but ήταν ζεστός (ζεστός-ζεστή-ζεστό is an adjective meaning hot) is not used in greek (you must specify who was hot), instead we say: έχει ζέστη, έχει κρύο (it=the weather has heat, has cold). Also you can say κάνει ζέστη/κρύο=literally, the weather does heat/cold.