May vs. Must

'May' and 'must' are modal verbs that are confused by learners because they talk about possibilities and probabilities. Click here to learn their differences.

What Is Their Main Difference?

The main difference between modal verbs 'may' and 'must' is that 'may' talks about possibilities and 'must' talks about necessities.

'May' is a modal verb mainly used to talking about giving and asking for permission,possibilities, and making offers. Have a look:

I may quit my job.

May I borrow your book?

'Must' is a modal verb that is mainly used to show that something is a necessity. It is also used to talk about likely events and suggestions. For example:

You must leave right now.

You must be tired.

Similarities

Possibilities and Probabilities

'May' and 'must' are used to show possibilities and probabilities. 'Must' talks about a great chance of occurrence while 'may' talks about a lesser chance of occurrence. For instance:

She may have bribed the judge.

She must have bribed the judge.

With Conditionals

Modal verbs such as 'may' and 'must' can be used as conditional verbs. Conditionals are sentences that show the occurrence of one event depends on the occurrence of another action or event. The table below gives an overview of 'may' and 'must' with all conditional types:

	May	Must
Conditional Type 1	✓	✓
Conditional Type 2	✗	✗
Conditional Type 3	✗	✗
Zero Conditional	✗	✗

Conditional Type 1

'May' and 'must' can be used in conditional type 1. This type shows a condition and the results that follow. These conditions are real situations with a high chance of occurrence. For example:

If you are exercising every day, you may lose weight.

If you are exercising every day, you must lose weight.

Negation and Question

Modal verbs are used as the head of their sentences and therefore they are changed in terms of form or place when we want to change the sentence to negative form or interrogative form.
To make a sentence negative, we add 'not' to the modal verb as shown below:

May → May not → Mayn't
Must → Must not → Mustn't

Take a look at the following examples:

I may attend the party. → I may not attend the party.

You must leave before 6 p.m. → You must not leave before 6 p.m.

To turn them into question form, we invert the modal verbs with subjects. Take a look:

I may attend the party. → May I attend the party?

You must leave before 6 p.m. → Must You leave before 6 p.m?

With Other Modals

We only have one modal verb in our sentences and we cannot use more than one modal verb at once. Take a look at these incorrect examples:

I ~~may~~ ~~could~~ look after the kids.

She ~~must~~ ~~should~~ keep the files safe.

Differences

Talking about Permission

We use 'may' to give and ask for permission. To ask for permission, 'may' is followed by 'I' or 'we.' Take a look at the following dialogue for clarification:

May I join you?

You may join us.

Talking about Obligation

We use 'must' to talk about actions and events that their occurrence is a necessity and perhaps a duty. For instance:

You must boil canned food before consuming it.

You must report any kind of abuse to the director.

Making Offers

Offers are statements used to show our willingness to do something. We use 'may' to make such offers. In this form, 'may' is followed by first-person singular or plural pronoun (I and we). For instance:

May I hold your hand?

May I invite you to dinner?

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