In could and would there are examples of requests. So how we will know where to use could and where to use would?

Would - We use would for several purposes: a) to talk about the past. b) to talk about future in the past. c) express the conditional mood. d)express desire. e)polite requests and questions. f)opinion or hope. g)wish and regret. The basic structure for would is: subject+ would+ main verb  eg. I  would like tea. (I -subject, like-main verb)        She would not go. Note that the main verb is sometimes in the form: have + past participle (She would have gone)be + -ing (She would be going)would for the past We often use would as a kind of past tense of will or going to: Even as a boy, he knew that he would succeed in life.Using would as as a kind of past tense of will or going to is common in reported speech: She said that she would buy some eggs. (She said,"I will buy some eggs.")The candidate said that he wouldn't increase taxes. (The candidate said, "I won't increase taxes.")Why didn't you bring your umbrella? I told you it would rain! ("It's going to rain.")We often use would not to talk about past refusals: He wanted a divorce but his wife would not agree.We sometimes use would (rather like used to) when talking about habitual past behaviour: Every weekday my father would come home from work at 6pm and watch TV.We would always argue. We could never agree. When talking about the past we can use wouldto express something that has not happened at the time we are talking about: would for the future in past: In London she met the man that she would one day marry.would for conditionals We often use would to express the so-called second and third conditionals: For those unaware, Second conditonal is used to talk about unlikely situations.   Eg. If I had millions dollars, I would give a lot to charity. Third conditional is used to talk about impossible conditions; impossible because these things happened in the past and you cannot change the past. Eg. If I had worked harder at school, I would have got better grades. Using the same conditional structure, we often use would when giving advice: I wouldn't eat that if I were you.If I were in your place I would refuse.If you asked me I would say you should go.Sometimes the condition is "understood" and there does not have to be an "if" clause: Someone who liked John would probably lov e John's father. (If someone liked John they would probably lov e John's father.)You'd (you would) never know it. (for example: If you met him you would never know that he was rich.)would for desire or inclinationI would lov e to live here.Would you like some coffee?would for polite requests and questionsWould you open the door, please? (more polite than: Open the door, please.)Would you go with me? (more polite than: Will you go with me? would for opinion or hopeI would have to agree.I would expect him to come.would for wishI wish you would stay. ( meaning - I really want you to stay. I hope you will stay.)They don't like me. I'm sure they wish I'd resign.

Could- We use could t0 : a)to talk about possibilities/abilities to do things in the past. b)to make requests. Use of could for past possibility or ability We use could to talk about what was possible in the past, what we were able or free to do: I could swim when I was 5 years old.My grandmother could speak seven languages.When we arrived home, we could notopen the door. (...couldn't open the door.)Could you understand what he was saying? could for requests: We often use could in a question to ask somebody to do something. The use of could in this way is fairly polite (formal): Could you tell me where the bank is, please?Could you send me a catalogue, please? Could have and Could have been are the "Second conditional simple" and "Second conditional Progressive" tenses of the word Could.