How to use would or could in English? [duplicate]

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When should we use “can”, “could”, “will”, “would”?

I am not a native English speaker and this would/could thing always bothers me. Please give some example + explanation on this. Sorry if the question is too naive.

"Would" and "could" really cover different situations, although both can be used to make a request.

"Could" generally denotes:

• Possibility (either "basic" posibility on a particular occasion, or "supposed" possibility): "It could rain this afternoon", "They could be planning an attack"
• Equivalent of "can" (possibility) in the indirect speech: "He said he could come".
• To formulate a request: "Could you get the shopping for me?"

Note that "could" doesn't tend to denote a possibility that is "generally true"; in this case, "can" is used (contrast "it could rain this afternoon" with "it can rain heavily this time of year").

On the other hand, "would" generally denotes:

• The result of a condition being fulfilled: "I would spend more time with my children, but I have to work"; "it would be great if you could come"
• The notion of future-in-the-past, effectively the equivalent of "will" in indirect speech: "He said he would come"
• In the negative, to denote a refusal in the past: "He wouldn't help me" = "He didn't want to help me"; "The car wouldn't start" = "The car refused to start". But note that this is just in the negative: people don't say "*He would help me" to mean "He offered/accepted to help me".
• To formulate a request: "Would you come here a minute?"
• In slightly literary use, an equivalent of "used to": "On Saturdays, she would spend the whole day staring out of the window"

"If I could, I surely would..." -Aerosmith song.

Could refers to ability while Would refers to willingness. Sometimes you can do something, but are not willing to do it, for example.