When should you use l'un and when should you use un? What's the difference? Well, there are good reasons for this syntactical variation. Remember, French is rich in syntax, so what might seem like a minor difference in structure can translate into a major difference in sound or meaning. The difference between the two forms is fairly simple; it has to do with grammar and register, or the level of formality or informality of the surrounding language.
'L'Un' as a Pronoun
In formal French, when un functions a pronoun, rather than an article or number, it can be replaced by l'un. 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, un is either a number (one) or an article (a, an).
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
'L'Un' at the Beginning of a Sentence
When un is a pronoun at the beginning of a sentence, it is nearly always replaced by l'un, for reasons of euphony, or making pronunciation in this musical language as fluid and harmonious as possible.
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.
Expressions With 'L'Un'
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 > 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
'Un' as a Number or an Article
When un is a number (one) or an article (a, an), it should not be replaced by l'un.
J'ai un frère et deux soeurs.
I have one brother and two sisters.
Je vois une femme.
I see a woman.
C'est un Apollon.
He's an Adonis.
Un jour, ça sera possible.
One day, that will be possible.
Il est d'un drôle !
He's so funny!