Hi Pramod, yes these are different. The first sentence is the present tense (we know from the word 'can') so it is still possible to do the action. The second sentence is the past tense (could = past form of can) so it WAS POSSIBLE for the past action to be done, but it WASN'T done, and now it CANNOT be done (because the time has passed).

Yes. It is different. The first sentence implies it is still possible for it to be done. The second sentence indicates that the opportunity has passed. Hope that helps.