Il faut que vs. devoir

Question

Bonjour. I'm trying to get a feel for when to use/not use the subjunctive mood. One sentence on my quiz was: "Il faut qu'ils aient fini leur devoirs avant le diner" (sorry - my accents aren't working). This translates to "They need to have finished their homework before dinner."

Would it be an alternative to say: Ils ont du [accent] leur devoirs avant le diner [accent]" (which I believe translates to "They must have finished their homework before dinner.")

Can you please explain how these are different?

Accepted Answer

Bonjour Kate,
What a great question! ;-)
These two statements aren't equivalent:
Il faut qu'ils aient fini leur devoirs avant le dîner
 -> using "il faut que + [Subjonctif]" (in Le Présent) means that there's an absolute need/condition to fill in before they can dinner
Ils ont dû finir leur devoirs avant le dîner 
-> using "avoir dû + [faire quelque chose]" gives a slight feeling of uncertainty 
I hope this is helpful.
Bonne journée !