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?