Both of the sentences above are translated as "J'ai du le faire".
BUT the two formulations in English have not-very-subtle differences in meaning.
"I had to..." implies "I was obligated to.." or "I was forced to..." - very definite!
"I must have..." implies "I may have forgotten to ..." -- quite indefinite!
How are these different flavors of meaning expressed en francais?
To avoid the confusion between possibility and obligation you can always use another verb that indicates obligation -
Il a fallu que je le fasse = I had to do it ( I had no other choice)
J'ai été obligé/e de le faire = idem
A couple of sentences illustrating this situtation -
J'ai dû perdre mes clés au marché = It's quite probable that I /I must have /lost my keys in the market
Il a fallu que je retourne au marché pour voir ...= I had to go back to the market to look ...
Hope this helps!
The distinction in French is less clear than it is in English. Often, to express that one simply had to do something, one uses the imperfect.
I had to do it for myself. -- Je devais le faire pour moi-même.Sorry, I just had to do it. -- Désolé, je devais simplement le faire.
I know, this then causes some ambiguity with the "should have" meaning of devoir used in the imperfect. To be absolutely clear, one needs context or use a different expression altogether.
To say "I should have done it", you can use the conditional:
I should have done that. -- J'aurais dû faire ça.
Expressing "I must have done that", normally uses the passé composé:
I must have done that. -- J'ai dû faire ça.
Again, in French there's less of a clear-cut distinction between those different meanings than in English. One just has to live with it. I'd be interested to learn how a native speaker sees this.
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