Student or Learner
I beleive can is possibility and could is ability.
Could is also used as past tense of can.
Which one is correct between these two?
1. Can you tell him about this.
2. Could you tell him about this.
When should each of it be used?
Just to add that your questions:
1. Can you tell him about this?
2. Could you tell him about this?
are not connected to ability, possibility, or tense. Can & could are used in requests also, which is what your examples are. Harry Smith is right!