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Accusative makes the object definite. Sometimes we prefer using an indefinite object sometimes not( if verb allows us)

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Bir şey bilmiyorsun. You don´t know anything. (indefinite)

Bir şeyi de bil. Know at least one thing. (definite)

I am not sure but

1)when we use "bir şey" as anything or something, we can´t use it with accusative.

2)when we think it as "a thing or one thing" we can use it with accusative.

I wonder what others think