Must: forms
Affirmative (+) form
Mustcomes first in the verb phrase (after the subject and before another verb):
Shemusthave lots of friends.
Mustcan’t be used with another modal verb.
Thismustbe your sister.
Not:This must can be your sister. orThis can must be your sister.
Negative (−) form
The negative form ofmustismustn’t. We don’t usedon’t/doesn’t/didn’twithmust:
Theremustn’tbe any rubbish left.
Not:There dosen’t must be any rubbish left.
We can use the full formmust notin formal contexts or when we want to emphasise something:
Youmust notleave any rubbish.
See also:
Modal verbs
Question (?) form
The subject andmustchange position to form questions. We don’t usedo/does/did:
Mustyou make that noise?
Not:Do you must make that noise?
We can usemustandmustn’tin question tags though tags withmustaren’t very common:
The house must be worth millions,mustn’tit?
Must: uses
Deductions and conclusions
When we think carefully about facts, we often usemustto express deductions and conclusions from these:
[fact]He’s so small.[deduction/conclusion]Hemustbe no more than four years old.
[Two teachers talking about a student]
A:He falls asleep in class every morning. (fact)
B:Hemustbe out late every night or maybe he works at night. (deduction/conclusion)
We usecan’t/cannotas the negative ofmustto deny something or make negative deductions or conclusions:
It justcan’tbe true. Hecan’thave left his job.
Thatcannotbe his sister. She looks so different.
See also:
Can
We usemust have + edform andcan’t have+edform to talk about deductions in the past. They always refer to deduction, not obligation:
[A wanted to talk to B so she phoned him but he didn’t answer the phone. She phoned again the next day]
A:I called you yesterday around three o’clock but youmust have beenout.
B:Wemust have beenin the garden. That’s a pity.
[A is telling B about his illness]
A:I spent a month in hospital before I was able to walk.
B:Thatcan’t have beeneasy for you.
In speaking, we very often express our reaction to what we hear using phrases such asthat must beorthat must have been:
A:She lives in Thailand now.
B:That must beamazing!
A:Twelve years ago Kevin and I went on a six-week camping trip.
B:That must have beenfun.
Obligation and necessity
Mustexpresses strong obligation and necessity:
Imusttalk to you about the new project.
Seat beltsmustbe worn even in the back of the car.
Theremustbe a minimum of two members of the company at the meeting.
We usehad tonotmustto express obligation and necessity in the past:
By the time we got back to our bikes, it was dark and wehad tocycle home in the dark without any lights …
Not: …it was dark and we must cycle home in the dark…
Last year, teachershad tomake a report on each child every week.
Not:Last year, teachers must make a report…
We usemustto talk about the future in the past when we report speech or people’s thoughts in formal contexts:
[Extract from a novel]
The pain was back in full force, but she knew shemustnot give in to it. Shemustgo on day by day.
We usewill have tomore thanmustto express future obligation, especially when talking about obligations at a particular point in the future:
He’ll have towait five weeks for his eye operation. Then he’ll have tohave both eyes operated on.
We often usemustwith more general references to the future, particularly when talking about obligations that come from the speaker:
The Prime Ministermustdecide in the next month.
Imusttry harder next time.
Imustpop round one evening next week.
When we talk about no obligation, we use eitherneed not,don’t/doesn’t/didn’t have toor the negative of the main verbneed(don’t/doesn’t need):
Youneedn’tworry about it. I’ll take care of it.
Youdon’t have toworry about it. I’ll take care of it.
Youdon’t need toworry about it. I’ll take care of it.
Rules and laws
We usemust notto talk about what is not permitted:
Youmust notpark outside the entrance.
Youmust notmake noise after 9 o’clock.
Mustandmust notoften occur in public signs and notices indicating laws, rules and prohibitions:
[airline website information]
All passengersmustpresent valid photo identification at check-in for all flights.
[bus company website notice]
Ticketsmustbe retained for inspection, andmustbe produced for inspection on request by any authorised official of Bus Éireann.(Bus Éireannis the name of the Irish national bus company)
See also:
Commands and instructions
Invitations and encouragement
We also usemustto express polite invitations or encouragement:
Youmustcome and see us soon.
Youmusttry some of this chocolate cake. It’s delicious.
Youmustgo and see that film.
See also:
Invitations
Criticisms
We use the question form ofmustin criticisms:
Mustyou keep playing that terrible music?
Whymustyou mispronounce my name every time?
Mustandhave(got)to?
Obligations
We usually usemustto talk about obligations which come from the speaker and we generally usehave (got) towhen we refer to obligations that come from outside the speaker.
| The obligation is from me to buy new clothes. |
| The obligation is from the school to buy new clothes. |
Must notanddon’t have to/haven’t got tohave different meanings.
| We usemustn’tto talk about something which is forbidden. |
| We usedon’t have to/haven’t got towhen something is not necessary. It is not forbidden. |
Don’t have tocan sometimes be used to criticise someone or to tell them not to do something. This is less direct thanmust not:
Youdon’t have todrink all of the juice!(stop drinking the juice!)
Deductions
We can also usehave got towhen we make deductions or draw conclusions.Mustis more common thanhave (got) toin this meaning:
Thatmustbe a fake!
That picture hasgot tobe a fake!
See also:
Have got toandhave to
Must: typical errors
We don’t usemustto express obligation and necessity in the past. We usehad toinstead:
When she got home, shehad tocook dinner before everyone arrived.
Not:When she got home, she must cook dinner before…
We don’t usemustto make predictions about the future. We usewillinstead:
Don’t worry about our accommodation because I found a nice hotel whichwillbe suitable for us.
Not:Don’t worry about our accommodation because I found a nice hotel which must be suitable for us.
See also:
Will