Making suggestions: Could

Examples:

--What should we do tomorrow?
(a) Why don't we go on a picnic?
(b) We could go on a picnic.

Could can be used to make suggestions. (b) is similar to (a) in meaning; i.e., the speaker is suggesting a picnic.


--I'm having trouble in math class.
(c) You should talk to your teacher.

--I'm having trouble in math class.
(d) You could talk to your teacher. Or you could ask Ann to help you with your math lessons. Or I could try to help you.

Should gives definite advice. In (c), the speaker is saying: "I believe it is important for you to do this. This is what I recommended.
Could offers suggestions or possibilities. In (d), the speaker is saying: "I have some possible suggestions for you. It is possible to do this. Or it is possible to do that."

--I failed my math class.
(e) You should have talked to your teacher and gotten some help from her during the term.

--I failed my math class.
(f) You could have talked to your teacher. Or you could have asked Ann to help you with your math. Or I could have tried to help you.

Should have  gives "hindsight possibilities." In (e), the speaker is saying: "It was important for you to talk to the teacher, but you didn't do it. You made a mistake.

Could have offers "hindsight possibilities." In (f), the speaker is saying: "You had the chance to do this or that. It was possible for this or that to happen. You missed some good opportunities."

Might (but not may) can also be used to make suggestions (You might talk to your teacher.), but the use of could is more common.


Cheers!
Naeem