Learn Turkish: Yönetmen, şarkı söyleyenleri dinliyordu. - The director was listening to those who were singing.

This is a shortened way of saying "the people who sing" (şarkı söyleyen kişiler). The suffix -(y)en creates a relative clause meaning "who sings". When we drop the noun (kişiler), we can attach the plural suffix -ler directly to the verb to mean "the ones who sing" or "those who sing": söyleyenler.

Why is there an -i at the very end of söyleyenleri?

It is the accusative case suffix. In Turkish, the verb dinlemek (to listen to) takes a direct object. Because the director is listening to a specific, defined group of people ("those who were singing"), the word needs the definite direct object ending -i.

The root for "to listen" is dinle-. Why does it become dinli- in dinliyordu?

This happens because of a rule called "vowel narrowing." When a verb ends in -a or -e (like dinle-) and is followed by the continuous suffix -iyor, that final vowel narrows to -ı, -i, -u, or -ü. Here, the -e in dinle changes to -i, making it dinliyor.

Why did we use dinliyordu instead of the simple past dinledi?