Here is an excellent answer from down in the comments at http://forum.thefreedictionary.com/postst7130_Will-would---Can-Could---When-should-I-use-them--.aspx. Can expresses Oportunity, Ability or Permission in the present time: 1) If I study hard, I can pass that exam. = If I study hard (right now), I have a chance to pass that exam. 2) Can you speak russian? = Do you have the ability to speak russian? (righ now) 3) Go ahead, you can take my turn. = Go ahead, you have my permission to take my turn (in this very moment I'm giving you my permission). As you can see, in sentences 2 and 3, the ability or permission is for real in the present time, wich means you can do the actions in this moment. In sentence 1, the possibility is for a close future, but it depends directly on what you do right now (if you sudy now). You use can when the oporunity, ability or permission is in the present, or depends on something you do right now. Could expresses an Oportunity, Ability or Permission for a possible future: A) If I studied hard (in the case I decided to do it), I could pass this exam. B) Could you speak russian? (if... you were a russian) C) If I wasn't here for my turn, you could go ahead and take it. (you would have my permission to do it if I wasn't present, but righ now it's not the case, so you don't have the permission right now, it's only a possibility) In these sentences (A, B and C), the opotunity, ability or permission is for real only in a possible future that depends on things that are not a fact right now. You use could when you talk about a possible future.

Can/could refer to ability, permission, possibility, knowing how, etc. Will/would refer to the future (will) and the (unreal) conditional would. Will and would can sometimes mean the same as want or desire. Can and will are in present, but may refer to the future (will usually refers to the future). Could and would are either past tense or conditional tense. In the conditional, they are often used for polite requests in the form of questions. All modal verbs are followed by the main verb in the base form (the infinitive witthout "to"). Modal verbs have no infinitive, no base form, and no participle of their own.

Let me add the following that isn't quite apparent from the formal answer above. "Could" and "would" also carry a sense of conditionality, eg if this condition is met, etc. So often when asking for a favor, we use "could" and "would" even though "can" and "will" are grammatically acceptable. "Could" and "would" have a slightly more polite tone. It is as if one is saying, “if it pleases you" or "if it is convenient" at the front of the statement.