Difference between can & could [duplicate]

I'm a native English speaker and I've been doing some research into English grammar for a programme I'm working on. However, on looking into modal verbs, I've only just come to appreciate how subtle and complicated they are.

What is the difference between these sentences? What possible different response might one have to each?

• I can go to the cinema tonight.
• I could go to the cinema tonight.
• I may go to the cinema tonight.
• I might go to the cinema tonight.

Granted, the first of these looks more like a statement of ability or possibility than the three that follow. But in what ways does it differ from the others, since all suggest ability and possibility? And crucially, what distinguishes the other three.

Perhaps it's a matter of tone, of context, of the speaker... The question may appear quite simple but I haven't found a satisfactory answer elsewhere.

One answer looked quite useful in that it covered levels of confidence and certainty but it was talking about the potential state of something rather than the possibility for the person who is speaking. Are there any differences in that sense?

Another looked at these verbs used in question form (e.g., differences between 'Can I...?' and 'May I...?') but I'm not so interested in that for now.

Any thoughts appreciated.

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.

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.

The modals verbs are [can], [may], [will], [shall], [ought], [must]. All of them show historical change in usage and in meaning: preterite forms are no longer restricted to past tense (and present forms drop out of use), and meanings have shifted from non-modal meanings to modal meanings. Some short notes on the history of each verb might be helpful:

Ought was originally the preterite form of owe, but owe itself cannot be used as a modal. Owe started out as a verb meaning "to have".

Must is the preterite form of the verb mote, which originally meant "is permitted to". Mote fell out of use around the sixteenth century.

Should is the preterite form of shall, the latter of which is falling out of use as a modal. Shall originally means "to owe".

Would is the preterite form of will, the latter of which is used mostly to mark future tense, and not as a modal. Will originally meant "to want", still retaining this sense in archaic use.

Could is the preterite form of can, and both can be used as modals. Could can still be used with past tense meaning, but usually isn't. Can originally meant "to know (how)"

Might is the preterite form of may, and both can be used as present tense modals. It originally meant "to be strong, to have power".

For [can] and [may], the preterite forms are generally used with a greater degree of modal remoteness than are the present tense forms (as other answers note). See pp.46--7 (also sec.8 of ch.3) in Huddleston & Pullum's A Student's Introduction to English Grammar on the use of preterite forms to express modal remoteness.

Use of the past tense in a present or future action is for subjunctive or optative moods of speech.

The third conditional for "if I could".

For example,

• Tomorrow, if I asked you to help me, would you do it?
• I would like to let you know that your services are no longer needed.
• I could go to the beach with you.
• Would you like to kiss me?

The subjunctive could be used for

• Impossiblity
• Possible but unlikely
• Retrospective (postmortem what-if analysis)

The optative is similar in use to the subjunctive to denote

• choice and the consequence if such a choice is taken.
• contigency

The most well-known illustration that has been used for decades to teach subjunctive speech is:

If I were a bird I could sing all day.

The sentence meant to say,

It is impossible for me to be a bird. But what if, I were a bird.

It is not acceptable to say, due to the subjunctive situation,

If I am a bird, I will sing all day.

Imagine visiting someone's home while there is a huge snow storm and your boots are all mucky. Your host may say,

Can you take your boots off in the mud room before coming into the living room? If you take your boots off, that will make my day.

However, we normally say,

Could you take your boots off in the mud room before coming into the living room? If you took your boots off, that would make my day.

This optative mood mimics the impossiblity of taking your boots off. Your host is expressing politeness by implying - I realise it could be unlikely to take your boots off, or that it is very inconvenient to take your boots off. But if you did it, my having a happy day would no longer be unlikely.

Therefore, using the optative is a form of politeness in English speaking cultures.

An example of contingency is

If the plane went down, the oxygen mask would automatically drop down and you could pull out the life vest from under your seat.

Using subjunctive speech in such a situation is so much more reassuring to passengers than the flight steward announcing assertively

If the plane goes down, the oxygen mask will automatically drop down and you can pull out the life vest from under your seat.

As a flight steward, you would wish to express that it is highly unlikely that the plane would go down, but just in case.

Another example -

Sister (deterministic assertion): Tomorrow is a holiday. What will you be doing to spend your day?

Brother (optatively): I don't know. I could go to the beach.

Sister (contingency, in case you are going): I would like to join you.

Sister (politeness): Could I?

Brother: Of course.

Sister: Thank you.

Brother (unlikelihood): What could we possibly do to persuade dad to come with us to the beach?

Sister (unlikelihood): Would he know the way to the beach?

Brother (uncertainty): He might know how to use Google Maps.

Sister (impossibility): If he had a magic carpet, he should use it!

I would say modals are not black or white grammar – right or wrong – but grey grammar – it depends!

The choice of a modal auxiliary depends on how polite you want to be, on the type of relationship the speaker and the listener are in.

It is a mirror of society and even a mirror of the changes society is undergoing.

So, for example, 'May I?', etymologically means asking permission, asking someone who is in power to grant or refuse you permission to do or get something.

As we live in a world that is more democratic than it used to be, hierarchical relationships are resented and played down, and using 'May I?' is felt to be humbling if not humiliating, if you are not a little child.

So, we tend to use 'Can I?' instead, which, etymologically, means asking the listener whether or not it is materially possible for me to do or get something… and is a little rude, as it kind of shoves the listener and his subjectivity aside to make way for objective conditions!

A way of softening the blow that shatters the authority of the listener is to use 'Could I?', where the conditional (would it be materially possible for me to do or get something, do you think?) restores the listener to some of its former glory