Learn Turkish: Bu, telefonun şifresi değil. - This is not the phone's password.

This is a definite noun compound. The owner (telefon) takes the genitive suffix -un to mean "the phone's". The owned item (şifre, meaning "password") takes the possessive suffix -si to show it belongs to the phone. Both words get a suffix to link them together.

Where does the -s- in şifresi come from?

The base word for password is şifre, which ends in a vowel. The possessive suffix here is -i. Because we cannot put two vowels next to each other, the buffer letter -s- is added to join them, making it şifresi.

Why is there a comma after Bu?

The comma separates the subject Bu (This) from the rest of the sentence. Without it, Bu telefonun şifresi would combine to mean "this phone's password" instead of "This is the phone's password".

Why is değil used here to mean "not"?

In Turkish, değil is the specific word used to negate nouns and adjectives. Because şifre (password) is a noun, we use değil. If we were negating an action (a verb), we would use a suffix like -me or -ma instead.