Learn Turkish: Tezi yazdıktan sonra dinleniyor. - After writing the thesis, he is resting.

It uses the pattern for "after doing something": verb root + -DIktAn sonra. For the verb yazmak (to write), we take the root yaz- and add -dıktan (following vowel harmony). The word sonra (after) always requires the "from" ending (-dan or -den) on the word before it, which is built right into this -dıktan suffix.

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

The -i is the accusative case ending, which marks a specific direct object. Because the person is writing a specific, known thesis ("the thesis"), the word tez becomes tezi. If they were just writing a general or unspecified thesis, it would just be tez yazdıktan sonra.

How do we know who wrote the thesis? There is no subject pronoun attached to yazdıktan.

In Turkish, if you don't put a specific person's ending on the -DIktAn part, we assume the person doing the action is the exact same person doing the main action at the end of the sentence. Since the main verb is dinleniyor (he/she is resting), we automatically know that same "he/she" is the one who wrote the thesis.

Is the verb dinleniyor related to the verb dinlemek (to listen)?