To keep it simple, I answer you without complex grammatical terminology. There are five possible situations of using can. 1. Ability In the first situation, we use can with a meaning of ability. For example, "I think I can lift the box" means that the speaker thinks that she/he is able to lift the box. The past tense form of the sentence is "I thought I could lift the box". 2. Permission In the second, we use can with the meaning of permission. Undoubtedly, all permissions are questions. Example: "Hey Jim, can I use your PC for awhile?". Use could for more polite forms. Example: "Could you please allow me speak?". 3. Request In the third case, we use it as a form of request. Example: "Can you please write it for me?". Use could for more polite form. Example: "Could you lend me $100?". 4. Possibility Sometimes, can is also used to mark a possibility. Example: "Using mobile phones while driving can cause accidents.". Use could if the possibility is uncertain. Example: "He could arrive later." 5. Offer When offering help to someone, use can. Example: "Can I open the bottle for you?". Could is unusual, formal, and archaic here. Could is used in two more ways where can isn't normal. 1. To make suggestions Example: "We could go out for awhile, if you like." 2. To express, forcefully, what someone must do Example: "You could speak up!" I hope that this answer could help you a lot! :)