I think I have seen could in present tense, "Could you please buy me some fresh apples?" is this usage correct or not?
Could is possible in present time for two scenarios.
Firstly for polite requests (formality).
Could you tell me the time?
is more formal than
Can you tell me the time?
Secondly for unreal scenarios (probability).
If I could come...
is less probable than
If I can come...
Could for Ability is not possible in the present, Can must be used.
Last year I couldn't swim, now I can.