Learn Turkish: Evin pencereleri açık. - The house's windows are open.

This is a definite noun compound, which shows ownership. In Turkish, both words in the relationship get a suffix. The owner, ev (house), gets the genitive suffix -in, becoming evin (the house's). The owned item, pencereler (windows), gets the 3rd person possessive suffix -i, becoming pencereleri. Together, they lock into the phrase Evin pencereleri.

Why doesn't pencereleri have the buffer letter s? Shouldn't it be penceresi?

If it were just one window, you would be exactly right: pencere ends in a vowel, so it would take the buffer s to become evin penceresi (the house's window). However, in this sentence we have multiple windows. We must first add the plural suffix -ler to make pencereler. Because pencereler ends in a consonant (r), it only needs the vowel -i for its possessive suffix.

Could I just say Ev pencereleri to mean the same thing?

No, leaving off the genitive suffix -in changes the meaning. Ev pencereleri is an indefinite compound that means "house windows" in general (a type of window). By using Evin pencereleri, you are talking about the windows belonging to one specific house.

Where is the word for "are" in this sentence?