This is the 12040th most frequent Turkish word.
göndermeyen
"Not sending" or "who/that does not send".
Here, 'göndermeyen' is used as an adjective modifying 'arkadaşımı', meaning 'the friend who did not send' the letter.
Mektubu göndermeyen arkadaşımı aradım.
I called my friend who did not send the letter.
In this sentence, 'göndermeyen' serves as an adjective for 'kişiyi', meaning 'the person who did not send' the specific item or letter.
Göndermeyen kişiyi kim bulabilir?
Who can find the person who did not send it?
The word 'göndermeyen' acts as a descriptor for 'öğrencilere', implying the students who did not send the designated item.