Look at these uses of en:
J'ai quatre chiens. - J'en ai quatre.
I've got four dogs. - I've got four of them.
Il a regardé deux films. - Il en a regardé deux.
He watched two films. - He watched two of them
Laure apporte cent roses. - Laure en apporte cent.
Laure is bringing a hundred roses. - Laure is bringing a hundred of them.
Vous voulez une boîte de chocolats ? - Oui, je vais en prendre une.
Do you want a box of chocolates? - Yes, I'm going to take one.
En as a pronoun can replace a noun when given a quantity of it = I bought twenty of them / I have three of them...
Note the order is as such:
Simple tense : en + [verb] + [number]
Close Future : aller + en + [infinitive] + [number]
Compound tense : en + [auxiliary] + [past participle] + [number]
This also applies to un / une (one):
Tu achètes un pantalon ? - Oui, j'en achète un.
Are you buying (a pair of) trousers? -Yes, I'm buying one.
Tu as des sœurs ? - Oui, j'en ai une.
Do you have sisters? - Yes, I have one.
as well as un autre / une autre (another one):
Tu as aimé ta glace ? - Oui, j'en veux une autre !
Did you enjoy your ice cream? - Yes, I want another one!
Mon stylo s'est cassé alors j'en ai acheté un autre.
My pen broke so I bought another one.
See also En can replace de + phrase (adverbial pronoun)
and Replacing nouns with le, la, l', les = it, him, her, them (direct object pronouns)
Learn more about these related French grammar topics
Examples and resources
Tu as aimé ta glace ? - Oui, j'en veux une autre !
Did you enjoy your ice cream? - Yes, I want another one!
Tu achètes un pantalon ? - Oui, j'en achète un.
Are you buying (a pair of) trousers? -Yes, I'm buying one.
J'ai quatre chiens. - J'en ai quatre.
I've got four dogs. - I've got four of them.
Il a regardé deux films. - Il en a regardé deux.
He watched two films. - He watched two of them
Vous voulez une boîte de chocolats ? - Oui, je vais en prendre une.
Do you want a box of chocolates? - Yes, I'm going to take one.
Laure apporte cent roses. - Laure en apporte cent.
Laure is bringing a hundred roses. - Laure is bringing a hundred of them.
Mon stylo s'est cassé alors j'en ai acheté un autre.
My pen broke so I bought another one.
Tu as des sœurs ? - Oui, j'en ai une.
Do you have sisters? - Yes, I have one.
Q&A
Andy
Kwiziq community member
4/06/16
Laura
Kwiziq language super star
6/06/16
Andy
Kwiziq community member
6/06/16
Dragana
Kwiziq community member
7 March 2018
2 replies
Can I say - As-tu un examen dans ce sujet? Oui, j'en aurai un.
Chris
Kwiziq community member
7 March 2018
7/03/18
I would say "As-tu un examen sur ce sujet?"
-- Chris (not a native speaker).
Aurélie
Kwiziq language super star
7 March 2018
7/03/18
Chris's sentence refers to a topic. If you mean a school subject, I'd use the word "une matière" instead:
"As-tu un examen dans cette matière ? Oui, j'en ai un fin mars."
Bonne journée !