ANSWER: This question was posted on my forum, and it began a discussion on modals. I first explored differences in meaning. I listed uses of would, including future in the past (You promised you would tell me!) and habitual past (When I was a child, I would ask my grandmother to tell me stories.) Then I explained how could expresses past ability (When Grandma was younger, she could do a cartwheel!) Finally, I noted the overlap in making requests, showing how the two verbs could be interchangeable (Would you please help me?/ Could you please help me?) This is when the student asked if it was correct to assume that “would” is more polite than “could.”
A funny thing happens when you are asked to commit to an answer and put it in print for possibly a hundred or more pairs of eyes to read. You sometimes doubt what you had previously felt certain about. Are “would” and “could” equally polite? I thought so. I did some reflection… and I still think so.
I believe one factor that equalizes these two modal verbs is pronunciation. Either verb could become rude with a change in sentence stress and intonation. For example, compare “Could you please pick that up?” and “Could you please pick that up?!” (Click here to listen.) In these examples, the modal would can be used to perform the same function: (1) make a polite request with no demand or urgency/ (2) make an impatient request born out of frustration.
In terms of choosing between would and could, we can help students by explaining that certain structures suit specific situations best. Examples:
1. would you like = Would you like to sit down? (polite offer/ invitation)
2. would you mind = Would you mind moving down one seat? (tentative request with some worry that it will cause offense, inconvenience, etc.)
3. would it be possible = Would it be possible for you to pick me up on your way to work? (similar to would you mind, used to make an inquiry about a desired action/ request for a favor)
Can you think of any others?