Could points to ability or possibility. Would points to willingness or intention. In polite requests either fits, and the difference is slight, so both can work when you just want a courteous ask.
Could points to ability or possibility, while would points to willingness or intention. In polite requests either fits, and the difference is slight, so both can work when you simply want a courteous ask.
could versus would
What each one means
Could points to ability or possibility. Would points to willingness or intention. In polite requests either fits, and the difference is slight, so both can work when you just want a courteous ask.
What each one means
kind=comparison
Could you open the window? asks whether it is possible or convenient for the person. Would you open the window? asks whether they are willing to. Both are polite, and in everyday messages the difference is slight.
I could help you move on Saturday means you are free and able to. I would help you move means you are ready and willing to do it. In a casual offer, both sound natural, so choose by what you want to stress: availability or readiness.
In high school, I could do my homework on the bus says you had the ability on those rides. I would do my homework on the bus makes a definite claim that it actually happened, regularly.
The single boundary that changes the choice most often: requests. Could you send the file? asks whether sending is possible or convenient for them. Would you send the file? asks whether they are willing to do it. In everyday messages either sounds polite, so pick based on which question you actually mean.
That same pair shows the overlap. If you just want a polite ask, both work and neither sounds wrong. The choice starts to matter when the point is the outcome rather than the request: It would help a lot states the intended result, so could does not fit there.