There is this book I am reading, Men are from Mars, women are from Venus, by John Gray, which has some interesting passages:
"5. Use Correct Wording. One of the most common mistakes in asking for support is the use of could and can in place of would and will. "Could you empty the trash?" is merely a question gathering information. "Would you empty the trash?" is a request.
Women often use "could you?" indirectly to imply "would you?" As I mentioned before, indirect requests are a turn off. When used occasinally they certainly may go unnoticed, but persistently using can and could begins to irratate men."
What I want to point out here is that the passage above is not directly about language itself. It is directed to English native speakers, who know pretty well the different nuances among those modal verbs. Specifically it tries to advise women how to get along well with men by using the correct modal verb.
Certainly a careful understanding of this passage is useful for any ESL student.