Learn Turkish: Hangi arabayı kendin tamir ettin? - Which car did you repair yourself?

The word kendi is the root for "self". In Turkish, we must add possessive endings to it to specify whose self we are talking about. Here, adding -n (the 2nd person singular possessive) makes it kendin, meaning "yourself". If it were "myself", it would be kendim.

Why does araba (car) have the -yı ending here?

Because of the question word hangi (which), we are talking about a specific car. When a specific noun is the direct object of a verb (the thing being repaired), it needs the accusative case. Since araba ends in a vowel, we add the buffer -y- and the accusative suffix -ı.

Why are there two words, tamir and ettin, to mean "repaired"?

Tamir is a noun meaning "repair" or "fixing". Turkish often uses the helper verb etmek (to do/make) alongside certain nouns to turn them into action verbs. So, tamir etmek means "to repair". Here, etmek is conjugated in the past tense for "you" (ettin).

Since the sentence uses kendin, do I still need the word sen (you) at the beginning?