Learn Turkish: Kimse onu övmedi, bunun için söyleniyor. - Nobody praised him, for this reason he is grumbling.

The root is söyle (to say or tell). By adding the reflexive suffix -n, it becomes söylenmek. While you might guess this means "to tell oneself," it is actually the standard word for "to grumble," "to mutter," or "to complain" (because grumbling is like talking to yourself).

Why is the verb övmedi negative if kimse already means "nobody"?

In Turkish, negative words like kimse (nobody/anybody) always require the main verb to be negative. So literally, this sentence says "Nobody did not praise him." You have to use the negative suffix -me on the verb for the sentence to be grammatically correct.

Why is it onu instead of just o?

onu is the accusative (direct object) form of o (he/she). Because the person is the target being praised, they are the direct object of the verb övmek (to praise). The -n- is a buffer letter between the root o and the accusative suffix -u.

Why does bu (this) have the -nun ending before için?