B1 · Can, must, want
devoir for a guess
«must have» puts the past on the wrong verb here.
Devoir does not only mean must. With a state behind it, it becomes a guess: Il doit être malade is «he must be ill» — not an obligation to be ill, but an assumption. English does exactly this with «must», so the present tense costs nothing.
The past is where the two languages part. English keeps the modal in the present and puts the pastness on the infinitive: «he must *have* forgotten». French does the reverse — it puts the pastness on devoir and leaves the infinitive alone: Il *a dû* oublier. Same three ideas, opposite arrangement.
That mirroring is the whole difficulty of the page, and it produces a very recognisable mistake: il doit avoir oublié, which is «must» plus a perfect infinitive, copied straight from English. It exists as a French sentence and means something closer to «he is bound to have forgotten» in a formal register — it is not what people say.
The negated guess does not go through devoir at all. Instead of il ne doit pas être là, French says il n'est sans doute pas là — where English would say «he can't be there».
Die Form
| he must be | il doit être |
| she must know | elle doit savoir |
| he must have left | il a dû partir |
| that must be expensive | ça doit coûter cher |
| you must be hungry | tu dois avoir faim |
| he must have been asleep | il devait dormir |
Die Regeln im Einzelnen
devoir plus a state is a guess, not a duty.
Il doit être en réunion.He must be in a meeting.
In the past it is il a dû plus the plain infinitive.
Elle a dû rater son train.She must have missed her train.
For a negative guess use sans doute, not ne pas devoir.
Il n’est sans doute pas au courant.He probably doesn’t know.
An action after it means duty; a state means a guess.
Tu dois partir. / Tu dois être fatigué.You have to leave. / You must be tired.
Woran man es erkennt
- doit être
- a dû
- doit avoir
- devait
Beispielsätze
Il doit avoir une trentaine d’années.He must be about thirty.
Vous devez être fatigués après ce voyage.You must be tired after that journey.
Elle a dû se tromper de jour.She must have got the day wrong.
Ça doit faire vingt ans qu’on ne s’est pas vus.It must be twenty years since we last met.
Die typisch deutsche Falle
Aufgepasst
«He must have forgotten» has the modal in the present and the perfect on the infinitive. French moves the past onto the modal: il a dû oublier, with the infinitive bare. Copying the English arrangement gives il doit avoir oublié, which nobody says in conversation. The rule is short: whenever the English has «must have», the French has a dû.
«must have» becomes a dû: the past moves to devoir.
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