Thank you very much Theparser,it became clearer on my mind,what is the difference of ''could'' from ''may'' and might''.In grammar books they are same,but in the questions they are different,thank you so much for your kindness.
NOT A TEACHER
(1) Thank you for your kind note.
(2) You wish to know about the use of "could" (the past tense of'
"can") in questions.
(3) Do you have a copy of Mr. Michael Swan's
Practical English
Usage? Many students feel that they cannot live without this book.
(4) Mr. Swan devotes 8 pages to the use of can/could.
(5) Here are a few things I found regarding its use in questions:
(a) Do you think she
could be lying? (possibility)
(b)
Could I have some more tea? (request)
(c) "Can I ask you something?" More polite or formal:
Could I ask
you something? [I would use "could" if I were talking to my boss!!!]
BUT: the other person must answer: Yes, you
can.
(d) "Can I carry your bag?" "Oh, thanks very much."
Mr. Swan says if you want the offer to "sound less definite,"
you may use "could":
Could I carry your bag?
(e) He explains that we use
can and
could to ask or tell people to do
things:
Could you lend me five pounds until tomorrow? (More polite, more
formal or less definite than "can.")
Do you think you
could help me for a few minutes?
(6) Finally, this is
only my example:
(In a crowded cafeteria. There is only one empty seat)
You: Hey, dude. Can I sit here?
The dude: Yeah, no problem, man!!!
***
You: Excuse me. May/
Could I sit here?
A well-mannered lady: Why, yes, you certainly may/can.
Respectfully yours,
James