Modal Verbs: Should, Must, Might, and Could

“You should leave now” offers advice. “You must leave now” makes leaving necessary. “You might leave now” describes a possibility, while “You could leave now” can suggest an available option. Changing one word changes what the listener understands.

Should, must, might, and could are modal verbs: they work with another verb to express meanings such as advice, obligation, possibility, and ability. Each has more than one use, so the situation matters as much as the word itself. The comparisons below show how to choose the meaning you need and build the sentence correctly.

Should, must, might, and could at a glance

These meanings overlap, but the words are not interchangeable in every sentence. “You should call Eva” recommends calling her; “You could call Eva” presents it as one option. “Eva might call” is a guess about what she will do.

How to form statements, negatives, and questions

Use the modal followed by the base form of the verb, such as go, work, or be. Do not add an -s ending for he, she, or it, and do not put to directly after these four modals. Write “She must finish,” not “She musts finish” or “She must to finish.”

Common contractions include shouldn’t, mustn’t, and couldn’t; might not is a useful everyday negative form. Questions do not need do or does: “Should I wait?” is correct, while “Do I should wait?” is not. For a short answer, the main verb can be omitted: “Should I wait?” “Yes, you should.”

Should: advice and expectations

Use should to recommend an action or say what you think is the right thing to do. “You should check the address before you leave” gives practical advice. “We should apologise for the delay” expresses what the speaker considers appropriate.

The negative, shouldn’t, advises against something: “You shouldn’t send the email while you’re angry.” Compare this with “You mustn’t share the customer’s password,” which states a prohibition. Advice can be strong, but it does not automatically mean there is a rule.

When should means “I expect this”

In “The train should arrive at six,” nobody is advising the train. The speaker expects that arrival time, perhaps because it appears on the timetable. Similarly, “This password should work” expresses an expectation rather than a guarantee.

To identify the meaning, ask whether the sentence recommends an action or predicts an expected result. “You should restart the computer” recommends an action. “The computer should restart automatically” describes what you expect to happen.

Must: requirements and strong conclusions

Use must when you present something as necessary: “All applicants must complete this section.” It can also express a personal obligation, as in “I must return that library book today.” A time expression can give must a future meaning: “We must send the application tomorrow.”

Have to is also common for requirements: “We have to show our tickets at the entrance.” Do not treat must as exclusively personal and have to as exclusively external; official rules can use must too. For a closer comparison, see modal verbs for obligation.

Mustn’t does not mean “not necessary”

“You mustn’t print this document” means printing is not allowed. “You don’t have to print this document” means printing is optional. The difference can completely change an instruction.

Imagine an online workshop: “You don’t have to turn on your camera, but you mustn’t record other participants.” The first part gives participants a choice. The second removes a choice because recording is prohibited.

When must expresses a deduction

A deduction is a conclusion drawn from evidence. In “Her coat and bag are here, so she must be nearby,” must shows that the speaker feels confident about the conclusion. It does not describe a duty or instruction.

For the opposite conclusion, can’t is the usual learner pattern: “That can’t be Lena’s bag; hers is blue.” Use must for a strong positive deduction and can’t when you conclude that something is impossible. Keep this separate from mustn’t used for a prohibition.

Might: something is possible, but uncertain

Use might when you do not know whether something is true or will happen. “Omar might be in a meeting” suggests a possible explanation for his silence. “We might visit on Sunday” describes a plan that is not settled.

Might not means “possibly not”: “The shop might not be open yet.” It does not mean the shop is definitely closed. Likewise, “I might not attend” leaves attendance uncertain.

Both might and could can express a possibility: “The parcel might arrive today” and “The parcel could arrive today” are both possible. Neither word gives a fixed probability, and there is no universal percentage ranking between them. Context and tone help communicate how uncertain the speaker feels.

Could: possibilities, requests, and past ability

Could often introduces an option: “We could meet online instead.” It can also suggest an explanation: “The screen is blank; the battery could be flat.” In these sentences, could refers to the present or future, even though it is also used as a past form of can.

Make a polite request or ask permission

Use “Could you…?” to ask someone to do something: “Could you send me the updated timetable?” Use “Could I…?” to ask permission: “Could I use this chair?” These questions concern the current situation, not past ability.

A natural reply to “Could I use this chair?” is “Yes, of course” or “Yes, you can.” When granting permission now, can is the usual modal in the reply. “Yes, you could” may sound as though you are discussing an option rather than simply giving permission.

Describe a general ability in the past

“When I was a child, I could remember long poems” describes a general past ability. For an achievement on one particular occasion, usually use was able to or managed to: “Yesterday, I managed to recover the deleted file.” Managed to draws attention to the successful effort.

The negative couldn’t can describe a single unsuccessful attempt: “Yesterday, I couldn’t open the attachment.” This is why changing every past could to was able to would be a mistake. The British Council’s guide to past ability gives further practice with this distinction.

Talking about the past: modal + have + past participle

Use modal + have + past participle to discuss a past situation from your current point of view. Examples of past participles include left, taken, and forgotten. The modal still controls the meaning, so must have left and should have left do different jobs.

Should have can also describe expected completion: “The repair should have been completed by now; let’s call and check.” This does not necessarily mean the repair failed to finish. The context distinguishes an expectation from regret or criticism.

For a past requirement, use had to: “Yesterday, we had to leave early.” Compare “They must have left early,” which normally reports your conclusion about what happened. These are different meanings, not alternative ways to express the same past obligation.

Two short conversations

Planning a journey

Nina: “We could take the bus or walk.”
Leo: “We should take the bus. The meeting starts in half an hour.”
Nina: “It might be delayed in traffic.”
Leo: “Then we must leave now if we want to get there on time.”

Could introduces options, should recommends one, and might raises a possible problem. Must then presents leaving as necessary to meet their goal. The speakers are choosing meanings, not simply moving between different levels of politeness.

Looking for a missing document

Sam: “Could you check the shared folder?”
Priya: “It’s empty. Alex might have saved the file somewhere else.”
Sam: “I’ve found an upload receipt with a different folder name. He must have uploaded it there.”
Priya: “We should ask him for the link.”

The conversation moves from a request to an uncertain explanation, then to a stronger conclusion based on evidence. It ends with advice about the next action. Notice that could you check asks for help, while might have saved discusses the past.

Common mistakes to avoid

Keep the verb after the modal in its base form. Replace “She should goes” with “She should go,” and replace “He might to arrive late” with “He might arrive late.” When you need a question, move the modal before the subject: “Could he arrive late?”

Do not put two core modals together in standard English. Instead of “I will might join you,” write “I might join you tomorrow.” Instead of “She must can swim,” use “She must be able to swim” if you mean that swimming ability is required.

Check the intended meaning as well as the grammar. “You mustn’t bring lunch” forbids bringing it, whereas “You don’t have to bring lunch” says it is unnecessary. “I could finish the report tomorrow” presents a possibility; “I should finish the report tomorrow” may express an expectation or a duty, depending on the context.

Practice: choose the meaning that fits

Choose the correct option for each situation. In question 5, select both words if both fit. Read the answer explanations after you have made your choices.

1. Give advice, not a compulsory instruction: “You ___ check the spelling before sending it.” (should / must)
2. The rule forbids recording: “You ___ record this session.” (mustn’t / don’t have to)
3. Printing is optional: “You ___ print your ticket.” (mustn’t / don’t have to)
4. Make a polite request: “___ you move your bag, please?” (Could / Should)
5. Express an uncertain possibility: “The delivery ___ arrive this afternoon, but we don’t know yet.” (might / could / both)
6. Which sentence is correct? (She might joins us. / She might join us.)
7. Describe general past ability: “When I was eight, I ___ play the piano.” (could / must)
8. Report a successful effort on one occasion: “After several attempts yesterday, I ___ unlock the door.” (managed to / could)
9. Report a past requirement: “Yesterday, we ___ submit the form before noon.” (had to / must have)
10. In “The train should arrive soon; it left the previous station on time,” does should express advice or an expectation?
11. In “She must have read the email; she has answered every question in it,” does must have express a deduction or a past obligation?
12. Express regret about not checking: “I ___ checked the address. I went to the wrong building.” (should have / might have)

Answers and explanations

For your own practice, choose a situation you know, such as organising a meeting. Write one recommendation with should, one requirement with must, one uncertain possibility with might, and one option or request with could. Then change one sentence to a negative and check how the meaning changes.

For further explanation, consult Cambridge English Grammar Today on should, must, and could, may, and might, or the British Council’s guide to probability and deductions. To work on another topic, browse our English grammar lessons.