1. I studied math for 10 years, but I'm still not good at it.
2. I have studied math for 10 years, but I'm still not good at it.
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Which one is grammatical?
1 means you used to study it, but don't study it anymore, and are not good at it.
2 means that you studied math in the past and are still studying it, but still are not good at it.