This is the 16288th most frequent Turkish word.
yapmamıştır
"Yapmamıştır" means "He/She/It has not done [it]."
The word "yapmamıştır" here is used to indicate a negative past action in a definitive statement regarding Ahmet's completion of the homework.
Ahmet bu ödevi yapmamıştır.
Ahmet has not done this homework.
In this case, "yapmamıştır" expresses a past action that did not occur, emphasizing contrast with a current occurrence.
O bu hatayı önceki sefer yapmamıştır, ama şimdi yaptı.
He did not make this mistake previously, but now he did.
Here, "yapmamıştır" is employed to explicitly denote a failure to perform a specific task by the mentioned time, explaining a cause for subsequent issues.