When to use Devoir to express to need vs. to must have

Question

I am having a difficult time deciding when devoir is appropriate and when it is not. All the other applications, I am ok with. But if devoir implies "must have" why is a purse a necessity? Why not just Avoir besoin? And why is sleep NOT a necessity (or I may be getting this confused at this point). This is getting to be more of a guessing/memorization thing than an actual understanding thing. I see from the previous posts that this has been discussed ad infinitum so it's not just me. Any easy way to decide when to use devoir and when NOT to use it in this context?

Thanks

Accepted Answer

You can use either ‘devoir’ or ‘avoir besoin de’ followed by a verb infinitive when you want to express ‘need to do …’ - don’t sweat too much over which in this situation. 
‘Avoir besoin de….’ is very frequently used in French, even if it may sound a little more complex to English speakers to use.
Some important differences are
 1. “Avoir besoin de” does not translate to English in the sense of ‘must’ nor does it translate to ‘should’.  It always indicates ‘need of’ or ‘need to do’  (as covered in the lesson) - adjusted for tense, of course. 
2. “Avoir besoin de “ can be followed by an infinitive or noun, without change in meaning from the sense of ‘need’. 
3. Devoir changes meaning when followed directly by a noun - it translates to English in the sense of ‘owe’. Do not use ‘devoir noun’ unless you are sure you mean ‘owe’.  Avoir besoin de cannot be used in this context.
4. Perhaps complicating things a little more, if you want to say something along the lines of ‘I must have that dress’, meaning ‘I want that dress (at any price)’, you can simply say ‘Je veux cette robe (à tout prix)’.  It is not usual in French to say ‘need’ when ‘want’ is what is really meant - so ‘must have (to really want) something’ is not a ‘need’ situation !
Most languages have multiple ways of saying similar things - but you will get a feel for what is more common in various situations by reading and listening to lots of French.