French language Q&A Forum

One of the things I love about Progress with Lawless French is that in the Written and Dictation exercies, alternate answers are given so you can learn all the different ways to express a given passage. What I don't like, however, is that when a result comes up "Your answer matched mine", no alternate answers are given. I really miss that extra chance to expand my French that is provided in the alternate answers.

Un jour on nous voudrons acheter une maison. Qui est-ce que le function de mot on dans le phrase precedent? 

Can anyone explain what the difference between the passe compose and past perfect forms of devoir are? google translate shows them as being the same thing:

J'ai dû faire quelque chose     -> I had to do something

J'avais dû faire quelque chose  -> I had to do something

Similarly what is the difference between the future and conditional forms:

j'aurai dû faire quelque chose  -> I should have done something

j'aurais dû faire quelque chose -> I should have done something

Thanks!