Resources
Lecciones de gramáticaPor temasFlash cardsLecturasEjercicios de listeningGlosarioFalsos amigos¿Cuál usar?SufijosPrefijosDiscover your levelBlogMustvsHave to
"Must" is internal obligation or logical deduction; "have to" is external obligation imposed by others.
Side-by-side comparison
Obligation coming from the speaker themselves, or logical deduction about something. Emotionally stronger.
I must call my mother. I have not spoken to her in two weeks.
You must be exhausted after that long trip.
This restaurant must be good, there is always a line outside.
Obligation imposed by rules, laws, another person, or circumstances. Neutral and factual.
You have to wear a helmet when riding a motorcycle. It is the law.
She has to work on Saturdays this month. Her boss asked her to.
Do we have to pay in advance, or can we pay at the door?
Common mistake
The most serious slip is the negative: "You mustn't come" and "You don't have to come" mean opposite things. The other is the past: "I must go yesterday" is wrong, you say "I had to go yesterday".
Trick to remember it
Ask yourself who is giving the order. If it comes from you ("I must call her", you set the rule), it is "must". If it comes from outside ("I have to wear a uniform", the job requires it), it is "have to". Mind the past: "must" has none, so you use "had to".
Quick rule
"Must" = internal obligation. "Have to" = external obligation. "Must not" = forbidden. "Don't have to" = not required.
Did you know?
MUST has no past tense in modern English, it is a "defective" modal verb (with missing forms). For the past of obligation, HAVE TO in past is always used: "had to." "I had to work late yesterday", never "I musted work late." This makes HAVE TO far more versatile than MUST grammatically: it has a past (had to), future (will have to), present continuous (be having to), perfect (have had to), forms MUST cannot express. This is one of the reasons advanced student textbooks present MUST and HAVE TO together: they are complementary where one ends, the other continues.
Practise what you learned
Exercise 1 · Pick the right one
You _____ try this cake. It's absolutely delicious!
Exercise 2 · Pick the right one
In the UK, you _____ drive on the left. It's the law.
Exercise 3 · Form the word
Related word pairs
'Have to' expresses external obligation; 'need to' expresses functional or personal necessity.
'Must' indicates obligation (you have to do it); 'mustn't' indicates prohibition (you must not do it).
'Must' can express obligation or deduction; 'can't' (not 'mustn't') expresses the negative deduction.
"When" is for certain future events; "if" is for uncertain or hypothetical ones.
Bryan López
Founder of SpeakUP Academy