When to use il fallait que

Question

The question asks for a correct translation of this sentence "I must have heard a suspicious noise to get up." The answer given in the quiz is "Il faut que j'aie entendu un bruit suspect pour me lever." Why doesn't Il faut have to be in the past given that aie entendu is in the past? "Il fallait que j'aie entendu . . . ."

Accepted Answer

Bonjour Diane,
- If the main clause = Le Présent or Le Conditionnel Présent
          → subordinate clause = Le Subjonctif Présent
All the actions are happening simultenously:
Je souhaite que tu viennes = I wish you to come
Je souhaiterais que tu viennes = I would like you to come
Il faut que tu entendes du bruit = you must hear a noise
 
- if you want to point out that the action from the subordinate clause happened before the action from the main clause (being in Le Présent), then,
main clause = Le Présent → subordinate clause = Le Subjonctif Passé
Je doute que tu aies été invité par le roi = I doubt that you were invited byt the king
The action of ‘being invited’ happened before the action of ‘doubting’ was raised
Il faut que tu aies entendu du bruit = You must have heard some noise
The action of ‘hearing’ happened before the action of ‘necessity’ was completed/mentioned
 
Have a look at the two examples from this link on French Past Subjunctive.
I do recognise that the Subjunctive is a really tricky notion to understand. But I do hope this is helpful.
Bonne journée!

Answer

In English, too, "must" is in the present tense and "have heard" is in the past tense. It's similar in French. What you're proposing would sound like: I had to have heard...