Stands for something

Sitesurf 

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"n'importe quoi" is short of "il n'importe pas quoi", meaning "it does not matter what".

So the n' is a negative.

https://www.duolingo.com/comment/6389254

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je dois en acheter un = je dois acheter un des chapeaux (one of the hats)

As I said, "en" can replace a singular (uncountable) or a plural:

• il y a de la viande; je dois en acheter (some)
• il y a des carottes; je dois en acheter (several)
• il y a des chapeaux; je dois en acheter un (one of them)

https://www.duolingo.com/comment/344650

vouvoyer = address as vous, speak formally
tutoyer = be informal, be on familiar terms
https://www.duolingo.com/skill/fr/Verbs:-Infinitive-2