Hi Can is a modal auxiliary verb, and "could" is the past tense of it.There is no -s in the third person singular. e.g: She can speak French. We use "can" to say that something is possible, to ask for permission and to make requests and offers. Can is used in informal situations. e.g: Can we go to cinema? We also use it to talk about abilities. e.g: I can drive. "Can" indicates future when used with a time word. e.g: I can visit the doctor tomorrow. After "can" and "could" we use the infinitive without to. e.g: I could/can play tennis. We use "could" to say that somebody had the general ability or permission to do something. e.g: I could swim very well. We also use it for make requests, and it makes the requests more polite than "can" does. e.g: Could you please open the door? Hope this helps.

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! :)

Well if we want to talk about our abilities in the past, we use ''could'' For example: I could climb a tree when I was 10 But we use .''can'' while talking about our abilities in general. For example: I can play violin. Could is past form of can if we use it to tell ability. However, ''could'' has other usages which refer to present or future. If we want to ask permission we can also use ''could'' Could you open the window? Or we can use could to make ''deductions'' about present or past For example: She is driving a Ferrari. He couldn't be poor. (strong possibility)

It depends on the context.

In brief ,,could'' is the past tense of ,,can''