What is the difference between 'can', 'could', 'may' and 'might'?

Solution 1:

Note that could is the past form of can, and might is the past form of may.

Past forms of these words are used in subjunctive and conditional constructions.

I can go to the cinema is a statement that you are able to go without any external conditions being in the way. (But the statement stops short of making a commitment: namely that you will go to the cinema.)

I could go the cinema. has multiple interpretations. One is that it's an incomplete conditional thought. You could go to the cinema, if what? It can also be uttered by someone who is in the middle of making a decision. What should I do tonight? Hmm, I could go to the cinema.

Quite possibly, in this kind of reasoning, the speaker, to some extent, externalizes the internal conditions on which the decision hinges. It is not simply true that the speaker can go to the cinema, because that is only possible if he doesn't choose some other mutually exclusive activity for the evening which precludes going to the cinema. That may be semantic the basis for why the conditional-making past participle is used for such statements.

I could go to the cinema tonight or I could go clubbing. I know! If I catch an early movie I can go to the cinema and I can go clubbing.

Now about may. I may go to the cinema is very similar to I can go the cinema, but as a native speaker, you know the difference between can and may being that between ability and permission or possibility.

Furthermore, modern English, the semantics of can stretches to cover that of may (but only in the area of permission, rather than possibility). Children frequently ask grownups permission using can I rather than may I.

I may go to the cinema has at least two possible meanings. One is that the speaker's privilege for that outing depends on permission from some authority. I can go to the cinema can still imply that, depending on the context. For instance, it obviously does in My dad said I can go to the cinema tonight.

But I may go to the cinema also has another meaning: that of possibility, and it means that going to the cinema is on the speaker's short list of possible activities. If an adult states that he or she may go to the cinema, of course we assume this interpretation, and not that the adult has permission from someone else. And I might go to the cinema means approximately the same thing.

The difference between I could go the cinema and I might/may go to the cinema is that the former is associated with reasoning about conditions or alternatives, whereas the latter is just a statement of possibility. The former statement informs us about a decision-making process going on inside the speaker, whereas the latter statement informs us that it is possible that the speaker will later be found at the cinema.

Something might happen and something may happen are not exactly the same, because might is used when conditions are attached. For example, if you lean over the rail, you might fall is more correct than if you lean over the rail, you may fall because you may fall states a possibility which is not conditional on anything. The verb might can substitute for may in expressing a pure unconditional possibility, but the reverse isn't true.

Solution 2:

You have a lot of very good answers here, and in the answers to earlier questions. If they leave you rather more confused than when you came here, that’s all to the good: it’s the nature of the beast.

Just by way of helping you understand what you’re up against, I went to the database at the university where my wife is a graduate student and poked in “English modal verbs”. Restricting the search to accessible journal articles from peer-reviewed publications with full text online, I got 4,452 hits. And her library is not particularly well supported in the humanities.

One problem with modals is that they all have very large semantic fields. Another is that, as you are finding, those fields overlap to an appalling extent — not just the four you’re working with, it might almost be true of any four selected at random.

So it’s quite futile to try to draw bright lines (or even merely mildly fuzzy globes) around any modal and say “This is its core meaning”. A vague definition is useless for discriminating between the words, but a sharp definition excludes so much that it’s useless for predicting or interpreting usage.

Which modal is employed in any given instance is likely to be determined not by distinct semantic intent but by dialectal or idiolectal considerations; by precedent context, both factual and discursive; and (probably and unhappily) by unknown factors which for all practical purposes may be treated as random dice-rolls.

My advice would be to find two actual university-level grammar folks, one from English and one from Linguistics, and ask each to pick ten or a dozen articles/books/proceedings/whatever. Ignore the pitying smiles.

I had an English professor once (actually, he was my father, but he was an English professor and he taught me most of what I know about the language) who told me, when I raised a question much like yours, that English modals are like Cleopatra’s asp:

CLOWN: This is most fallible, the worm's an odd worm. [...] You must think this, look you, that the worm will do his kind. [...] Look you, the worm is not to be trusted but in the keeping of wise people; for, indeed, there is no goodness in worm.

I wish you joy of the worm.