English · A2 · verb · 4 minutes
Must: obligation and deduction
Use must + base verb for strong necessity and confident conclusions.
Understand the choice
Must can impose obligation or express a logical conclusion. Context decides whether You must be tired is deduction or You must leave is necessity.
Core rule: Must has one present form and takes a bare infinitive; must not prohibits, while do not have to removes necessity.
Notice it in context
We must leave now, but the children must stay here.
Debemos irnos ahora, pero los niños deben quedarse aquí.
Must is unchanged with we/children; leave/stay are base forms.
Passengers must show a boarding pass.
Showing the flight document is required.
Must expresses an external rule.
The lights are off; she must be asleep.
The dark lights support the conclusion that she sleeps.
Must be expresses confident deduction.
What to remember
- Had to supplies ordinary past obligation.
- Mustn’t means prohibited; needn’t/don’t have to means unnecessary.
Common trap: Do not confuse must not with do not have to.