Learn Turkish: Köpek onun arkasında koşuyor. - The dog is running behind him.

This is the buffer letter n that you are learning about in this lesson. The root word is arka (back or behind). First, we add the 3rd person possessive suffix -sı to make arkası (his back). Then, we want to add the locative suffix -da (at or in). Whenever you add a case suffix right after a 3rd person possessive suffix, Turkish requires the buffer letter -n- to separate them. So, arkası + n + da becomes arkasında (at his back, meaning behind him).

Why does behind him translate to onun arkasında using the locative case?

In Turkish, positional words like behind, inside, or under work like nouns. Arka means the back or the space behind. To say behind him, you literally say at his back. Onun means his, arkası means his back, and the -da suffix means at. Put together, onun arkasında literally translates to at his back, which is how Turkish expresses behind him.

Could I just say Köpek arkasında koşuyor and drop the word onun?

Yes! Because the suffix -sı in arkasında already tells us it means his, her, or its back, the pronoun onun is optional. You can drop it entirely unless you want to put special emphasis on exactly who the dog is running behind.

Why does use the vowel u before the suffix?