Learn Turkish: Kedi kapının önünde bekliyor. - The cat is waiting in front of the door.

In Turkish, spatial words like "in front of" or "inside" work like possession. You are literally saying "at the door's front." Kapının is "the door's", ön is "front", the -ü makes it "its front", and the -de means "at/in".

Where does the -n- come from in önünde?

That is a special buffer letter. Whenever you attach a location ending like -de (in/at) to a word that already has a third-person possessive ending (like the -ü meaning "its"), you must insert an -n- between them.

Why is the verb bekliyor instead of bekleyor?

The root of the verb "to wait" is bekle-. When the present continuous suffix -yor is added to a verb ending in the broad vowels -a or -e, that vowel narrows into -ı, -i, -u, or -ü. Here, the -e narrows to -i, giving us bekliyor.

Why does kapı get the ending -nın instead of just -ın?