Learn Turkish: Bozuk saati kendim tamir ettim. - I repaired the broken watch myself.

The word kendi means "self". To say "myself", "yourself", etc., Turkish adds possessive suffixes (the same ones you use for "my", "your"). Here, adding -m (my) makes kendim (my-self). You don't even need the pronoun ben (I) because kendim and the verb ending -m already make it clear who did the repairing.

The word saat has the back vowel "a". Why does it take the front vowel suffix -i to make saati?

Normally, vowel harmony rules say an "a" should be followed by an "ı". However, saat is an exception because it is a borrowed word. In Turkish, a few borrowed words have a "soft" pronunciation at the end, causing them to take front-vowel suffixes (like -i, -e, or -ten). This is why it is saati and not saatı.

Why do we need the accusative case ending -i on saati?

The suffix -i marks the specific direct object. Because you are talking about a specific broken watch ("the broken watch") that is receiving the action of the verb, it needs the accusative case. If you were talking about repairing a broken watch in general, you wouldn't use it.

Why is tamir ettim written as two words?