Can I use your mobile? -> more politely, less sure, in the present: Could I use your mobile, please? I can read this text now. -> in the past tense: I could read this text yesterday. Could is NOT always the past tense of can. See the first two example. What else would you like to know?

CAN is possibility in the present, and COULD is in the past. CAN -- I can fly. COULD -- When I was a child I couldn't fly.

Can : informal polite request : Can I borrow your pen? Could : polite request : Could I borrow your pen? Could is used for positive suggestions: We could go to the mall. Can is a physical ability not a decision. You can speak English. It is something you are able to do. Could you please speak in English? is a request or decision. Also you can use COULD for something you are 50% uncertain about. (The coin could land on heads). The plane could have landed an hour ago.