Modal Verbs: 'Can' vs 'May'

Can you use 'can' or must you use 'may'?

We all know the joke: a student raises their hand and asks the teacher “Can I go to the bathroom?” and the teacher responds, “I don’t know—can you?”

The “joke” here is based on the insistence that you should use 'may' when asking for permission to do something, and 'can' when speaking about ability. By this logic, the student should have said “May I go to the bathroom?” since their ability to use the facilities is likely not in question.

In reality, 'can' likely has more use in the “permission” sense than is recorded, as it is more informal and so shows up in speech more frequently than 'may' does. 'May' is the more formal word, and if you are at all concerned about being tut-tutted, a safe choice. 'Can' is now the verb of choice for ability, and both 'can' and 'may' are still used in the "possibility" sense. You may use 'can' if you wish, and you can use 'may' if it makes you feel better.

It was a choice of teachers and grammarians to proscribe that can should only be used of ability and may of permission. We find the rule spelled out clearly in Charles Lurie’s How To Say It: Helpful Hints On English (1926). There is no particular reason for the rule, except for the fact that 'may' has been used longer to mean “to give permission” than 'can' has. Nonetheless, the “rule” lives on.

 

More on merriam-webster.com

Both 'can' and 'may' are modal verbs.

MODAL VERBS

are used to express certain hypothetical conditions, such as advice, capability, or requests. Because they are a type of auxiliary verb (helper verb), they’re used together with the main verb of the sentence and they change its meaning slightly .

Consider the difference between these two examples: 

'I swim every Tuesday.' It is a simple factual statement. The speaker participates in a swimming activity every week on Tuesdays. 

'I can swim every Tuesday.' This example uses the modal verb 'can'. Notice how the meaning changes slightly: the speaker does not swim every Tuesday; he's saying he is capable of swimming every Tuesday if he needs to. It’s hypothetical. 

 The most frequently used modal verbs are: can - may - might - could - should - would - will - must. 

When are modal verbs used? See grammarly.com

USES OF 'CAN'

Ability

We often use can to talk about ability to do something in the present or future:

I can sing one song in Polish.

Can you sleep on your back?

We can go swimming after school tomorrow, if you like.

We often use can with verbs of perception such as hear, see, smell, taste, and mental process verbs such as guess, imagine, picture, understand and follow (in the sense of ‘understand’):

I can hear you.

I can see her coming down the road now.

Can you smell something burning?

I can guess why you’re angry.

We can’t follow these instructions for installing this new DVD player. (We can’t understand these instructions.)

General truths

We use can to talk about things which we think are usually, but not always, true:

Reducing cholesterol through diet can be difficult. (It’s not always difficult for everyone, but in general it is difficult.)

Fireworks can frighten pets.

Swans can be very vicious.

We don’t normally use could to talk about what we believe to be true in the present.

Compare:

Exercise can help reduce stress.	I believe this is a general truth or fact.
Exercise could help reduce stress.	I see this only as a possibility.
Finding a hotel in August can be difficult.	I believe this is a general truth or fact based on my experience or knowledge.
Finding a hotel in August could be difficult.	I see this only as a possibility.

Permission

We often use can to ask for or give permission:

Can I take Daisy for a walk?

Students can use calculators during the exam.

We use can’t to forbid (say what you must not do):

You can’t park there.

You can’t just take the day off work. You have to have permission in advance.

 Possibility

We use can to express possibility or to question possibilities:

We can go to Rome in June because both of us have a week off work. (It is possible for us to go to Rome because we don’t have to work in June.)

Well, how can you be on a diet if you buy so much chocolate? (I don’t think it’s possible that you are on a diet because you still buy lots of chocolate.)

Guessing and predicting
(can’t as the negative of must)

When we want to guess or predict something, we use can’t as the negative form of must. We use can’t have + -ed form as the negative form of must have + -ed. Can’t and can’t have + -ed form express strong possibility:

A:

Who owns this blue coat? It must be yours.

B:

It can’t be mine. It’s too big. (A uses must to guess that the coat belongs to B. He sees this as a strong possibility. B uses can’t to express strong negative possibility. The coat is too big, so it isn’t his.)

A:

Roy must have made a lot of money.

B:

He can’t have done. He doesn’t even own a house. (A makes a deduction that Roy has made a lot of money. B sees this as very unlikely and so expresses it as a negative possibility.)

 Requests

We use can as a question form to make requests:

Those cakes look so good. Can I try one?

Can I have your surname?

Can you help me with this form?

 Reproaches

We use can’t as a question form to ask people to stop doing something we don’t want them to do, or to do something they are not doing which we want them to do:

Can’t you stop making that awful noise?

Why can’t you just be nice to her instead of upsetting her?

Offers

We use can as a question form to make offers:

Can I help you lift that?

Can we do anything for you?

The past form of can is could. We use it to talk about ability in the past:

They could see a light on in the house as they drove past at 10 pm.

When we question the possibility of something in the past, we use can’t have + -ed form:

You can’t have arrived here earlier than me.

See https://dictionary.cambridge.org/grammar/british-grammar/can 

Today's laugh 😃

USES OF 'MAY'

Permission

We use may to ask for, give and refuse permission. It is quite formal.

asking for permission	giving permission	refusing permission
May I leave the room?	Yes, you may.	No, you may not.
May we use your phone?	Yes, you may.	No, you may not.

Can, could and may are all used to ask for permission. May is the most formal/polite and could is more formal and polite than can.

Possibility

We use may to refer to weak possibility in the present and future:

The economy may go up or down in the next year. (I think both are possible, the economy going up or the economy going down. I am not making either one a strong possibility.)

I think I may go to the doctor today and try to get some antibiotics. (I am not very sure yet if I will go to the doctor.)

General truths

We use may in formal writing, especially academic English, to describe things which the speaker thinks are generally true or possible. In this case, it is a more formal equivalent of can.

Compare:

A typical farmer’s cottage can be seen in the Ulster Folk Museum.	Both sentences express what the speaker believes to be a general truth about where the cottage is located. The speaker knows that there is a cottage in the museum and a visitor is able to see them there if they want to. May is more formal.
A typical farmer’s cottage may be seen in the Ulster Folk Museum.

Accepting a different view or opinion

We often use may to accept a different view or opinion, especially with well, and/or followed by but:

One month may well be too long to go away on holiday.

I may be wrong but I am going to tell the police about it.

The couch may well cost more but it’s going to be different.

See https://dictionary.cambridge.org/grammar/british-grammar/may

Making a polite offer

Like can, we can use may to offer do something for someone else, though it is generally a more polite, formal way of doing so:

    May I help you set the table?

    

    May we be of assistance in any way?

Expressing wishes for the future

May is also used in more formal language to express a wish or desire that something will be the case in the future. When used in this way, may is inverted with the subject:

    May you both have a long, happy life together.

    May you be safe in your journey home.

    We’ve had great success this year; may we continue to do so for years to come.

    May this newfound peace remain forever between our two countries.

As a rhetorical device

Sometimes, we use may in this way as a rhetorical device to politely introduce or emphasize an opinion or sentiment about something, in which case we invert may with the subject:

    May I just say, this has been the most wonderful experience of my life.

    May we be clear that our firm will not be involved in such a dubious plan.

    May I be frank: this is not what I was hoping for.

Note that we can accomplish the same thing by using the verbs let or allow instead:

    Let me be clear: this decision is in no way a reflection on the quality of your work.

    Allow us to say, we were greatly impressed by your performance.”

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