This is the 3113th most frequent Turkish word.
bıçağı
"The knife" (accusative form of "bıçak").
Here, 'bıçağı' is in accusative form, indicating it as the direct object of 'gördüm' (I saw).
Bıçağı masanın üzerinde gördüm.
I saw the knife on the table.
In this sentence, 'bıçağı' remains the direct object, used to specify the item being sharpened.
Ahmet bıçağı bilemeye başladı.
Ahmet started sharpening the knife.
The word 'bıçağı' highlights the object being moved or placed in the action of the sentence.