Learn Turkish: Patronun sekreteri nerede? - Where is the boss's secretary?

The -un suffix is the genitive case, which is how Turkish says "'s" or "of". It changes patron (boss) into patronun (the boss's). It uses -un specifically because the last vowel in patron is 'o', following the 4-way vowel harmony rules.

Why is there an -i at the end of sekreteri?

In Turkish, it is not enough to just put "'s" on the owner. The owned thing also needs a suffix! Because the owner (patron) is a third party (like he, she, or it), the word sekreter gets the third-person possessive suffix -i. Turkish literally phrases this as "the boss's, his/her secretary".

I'm asking "where is", but I don't see a word for "is" in this sentence. Why?

In Turkish, the "to be" verb (am/is/are) often doesn't need a visible word or suffix for "is" (the third-person singular). Just saying Patronun sekreteri nerede? (literally: Boss's secretary where?) is a complete, perfectly correct sentence.

Could I use nereye instead of nerede here?

No, because nereye means "to where?" and implies movement or direction (like "Where is the secretary going?"). Nerede means "at where?" and is used to ask about someone's current location, which is what we need in this sentence.