I think we should use “would”, instead of “could” ! Can this be confirmed by some English teachers ?
Yes, both are OK.
My English teacher explained to me that it is not appropriate to use “could”. She said the people I am talking to is always “capable” of doing the thing, but she may not want to do this for me. However, if I use “would”, there is an implication of a request made to the one I am talking to. And this is more appropriate. Is this explanation valid ?
No. ‘Could’ is a common polite response here, just as ‘would’ is.
As the teacher says, it’s correct to say either “could” or “would”; it’s the “should” that puts me off. My ex husband is English. He was forever writing business letters saying, “I should be grateful if you would…”. As an American, that looks strange to me. It seems as if the writer is saying, “I should be grateful, but I’m not.”
If you are writing a business letter in Britain, the best practice is to write, “I should be grateful”, just to be on the safe side, if you are unsure.
In the States, write, “I would be grateful”, only if you are writing a formal letter. Informally, it’s more acceptable (in the sense of being “the done thing”) to write, “I’d appreciate it if you would…”.