Well, without context I can't tell what the issue was so I can only guess: devoir expresses obligation where as besoin de expresses need, so you can use both with a verb, but only besoin de with a noun.
You can say:J'ai besoin de faire les courses. (I have to go shopping)Je dois faire les courses. (I need to go shopping)
And you can say:J'ai besoin d'un sac. (I need a bag)
But you can't say:Je dois un sac. (I have to the bag)
[Note: in fact, you could actually say that but it would mean "I owe a bag" which in a very specific context might actually be something you would say - a game of poker playing with bags, perhaps? So long as you know it doesn't mean need here...]
Is that helpful?
Sign in to submit your answer
Don't have an account yet? Join today
Test your French to the CEFR standard