I think you forgot something.
I think you have forgotten something.
Which is better? Is both correct?
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Idiomatically they mean basically the same. The difference is in the tense. The first is the simple past, it means that at some point in the past you forgot it. The second is present perfect, which means that you forgot in the past and are continuing to have forgotten up to now.
You could say "I forgot it, but now I remember", because the past simple tells you nothing about present condition, you cannot say "I have forgotten it but now I remember it" because the present perfect tells you about the range of time from the past all the way up to and including the present. (FWIW, you can also say "I had forgotten it, but now I remember." This is the past perfect and tells you the condition from one point in the past till another point in the past.)
The first tells us only that at one time it has been forgotten, the second that you forgot in the past and continue to forget even till today.
However, from a casual conversational point of view the two are largely used interchangeably.