why use the plural (les bois) in 'nous avoids fait une longue promenade dans les bois.'?

Hi B,

Because we don’t know which specific wood they are talking about you would use the plural 'dans les bois’ or the singular ‘dans la forêt’ ( in the woods).

But if you were talking about a specific one like ‘Le bois de Boulogne’, you would then use the singular ‘bois’

Hope this helps!

Note, that in English you could also say, "He took a walk in the woods." You wouldn't say "He took a walk in the wood", unless you were referring to a specific one.