This is the 2938th most frequent Turkish word.
seni
"You."
Here, 'seni' is the accusative form of 'sen', indicating the direct object (you) in the sentence.
Seni çok seviyorum.
I love you very much.
In this sentence, 'seni' is again the accusative form of 'sen', used here to represent the direct object of the verb 'saw'.
Seni dün parkta gördüm.
I saw you at the park yesterday.
'Seni' is in the accusative case, showing the person being thought about, as the complement of the verb 'thinking'.