Un vs L'un

What's the difference between un and l'un? Isn't the second one kind of redundant? Learn about when and why to use l'un in place of un in this lesson.

The difference between un and l'un is fairly simple - it has to do with grammar and register.

1) When un is a pronoun,* rather than an article or number, it can be replaced by l'un in formal French:

Tu dois choisir l'un de ces livres
You have to choose one of these books

J'ai vu l'un de ses amis
I saw one of his friends

*How do you know whether un is a pronoun, article, or number? Very simple: any time un is followed by a preposition, usually de, or by anything else other than a noun, it's a pronoun. Otherwise it's either a number (one) or an article (a, an) - these are usually interchangeable.

2) In addition, when un is a pronoun at the beginning of a sentence, it is nearly always replaced by l'un, for reasons of euphony:

L'un de mes meilleurs étudiants est à l'hôpital
One of my best students is in the hospital

L'un de vous doit m'aider
One of you has to help me

3) There are also a number of fixed expressions with l'un:

• c'est tout l'un tout l'autre - there's no in between, everything is black and white
   
• de deux choses l'une - there are two possibilities
   
• l'un à l'autre - to each other
   
• l'un après l'autre - one after the other
   
• l'un dans l'autre - all in all
   
• l'un d'eux, l'un d'entre eux, l'une d'elles, l'une d'entre elles - one of them
   
• l'un et l'autre - both (of them)
   
• l'un l'autre - one another, each other
   
• l'un ou l'autre - either one, one or the other
   
• ni l'un ni l'autre - neither one
   
• soit l'un soit l'autre - either one, one or the other

Note: When un is a number or article, it cannot be replaced by l'un:

   J'ai un frère
   I have one brother

   Je vois un homme
   I see a man