French language Q&A Forum

In the example, for actions already done, using the infinitive Passé:  "C'était très difficile, merci de m'avoir aidée."  It appears the usage demands an agreement (if the speaker in the case was female.)  Would it always be the case that agreement should be made?  

A light bulb went off for me after reading Sandra’s post below.

If I may add my two cents worth to expand on what she said…..

When I looked back over my incorrect answers, I had been choosing responses containing ‘devoir’ without an infinitive following, so in reality those sentences had an entirely different meaning.

Devoir + noun (no verb) = to owe 

Je dois de l’argent – I owe some money

Il me doit dix euros – He owes me 10 euros

So only the first two out of the three following test examples can be correct or mean "to need": 

•Marie doit ACHETER un nouveau sac à main = correct (devoir + infinitive)

Marie needs to buy a new handbag

•Elle doit RENTRER de bonne heure = correct (devoir + infinitive)

She needs to go home early

Vous devez un nouveau vélo = incorrect (no infinitive after devez)

I owe a new bike??

Am I on the right track here?

Question today was "Un jour, on _____ sur Mars."  In a previous   test, "On a ______ opinion."  The previous answer was "On a notre opinion," but today's test was said to be "On ira sur Mars."  How is one to know if the "on" refers to a singular person (on ira) or to several individuals (On a notre opinion)?  It's a bit confusing.  Please clarify.  Thanks.