Must or should?
Must removes the choice. Should leaves it with you. The negatives are further apart than the positives.
The short answer
Must states an obligation with no way around it, or a conclusion you are sure of. Should gives advice or says what is expected. Their negatives split completely: mustn’t forbids, shouldn’t advises against.
Which one do you need?
| Meaning | Form | Example |
|---|---|---|
| obligation, no choice | must | Passengers must wear a seatbelt. |
| advice, recommendation | should | You should wear a coat. |
| near certainty | must | She must be exhausted. |
| expectation | should | The parcel should arrive today. |
| forbidden | mustn't | You mustn't smoke here. |
| a bad idea | shouldn't | You shouldn't smoke. |
Practice the choice
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Getting your first question ready.
Side by side
Three sentences for each form, taken from the 225 sentences this drill uses. The answer is highlighted.
must
- There is nothing to discuss: children must obey their parents' rules.
- The traffic here is deadly: he must be careful while crossing the road.
- The exam allows no books: students must take proper notes during lectures.
should
- It's cold outside, so you should wear a jacket, though nobody will check.
- Nobody is forcing them, but children should be respectful towards their elders.
- No teacher insists, but you should listen carefully in class to understand the material.
Obligation: how much choice is left
Must leaves none. It is the word of rules, laws and notices:
- Visitors must report to reception.
- I must remember to call her.
Should leaves the decision with the listener. It is the word of advice:
- You should see a doctor.
- We should book early.
A useful test: if ignoring the instruction has a consequence imposed from outside, it is must. If ignoring it is simply unwise, it is should.
Certainty: how sure you are
Both modals also talk about likelihood, and the strength matches the obligation sense.
Must is a confident conclusion:
- The lights are off — they must be out.
- You must be joking.
Should is an expectation, weaker:
- The train should be here by now.
- It should work.
For the negative conclusion, must is replaced by can’t: they can’t be out, I just saw them. Mustn’t is not available for this job.
Mustn’t and shouldn’t are not neighbors
The two negatives mean genuinely different things:
- mustn’t = it is forbidden — you mustn’t park here
- shouldn’t = it is a bad idea — you shouldn’t park there, it’s a tow-away zone
And neither of them means “there is no obligation”. That is don’t have to, which is covered on its own page.
No past, no future
Must has only one form. For any other tense, English borrows from have to:
- I must go now.
- I had to go early yesterday.
- I’ll have to go early tomorrow.
Should behaves the same way in the past, with should have:
I should have called. — I didn’t, and it was a mistake
Must or have to
Both state obligation, and in most sentences either works. The shade of difference is where the obligation comes from:
- I must stop smoking. — my own decision
- I have to wear a tie at work. — someone else’s rule
In speech have to is far more common; must sounds formal, and in notices it is the standard form. See has to or have to for the agreement.
Mistakes to avoid
Wrong: You mustn't come if you're busy. correct: You don't have to come if you're busy.
Mustn't forbids. To say there is no obligation, English uses don't have to.
Wrong: He must to finish today. correct: He must finish today.
Modal verbs take the plain verb with no to.
Wrong: Yesterday I must leave early. correct: Yesterday I had to leave early.
Must has no past form. The past of obligation is had to.
Wrong: She musts be tired. correct: She must be tired.
Modals never take -s, whatever the subject.