The pronoun en
The partner of y, and the one English has no equivalent for at all — which is why it gets left out.
en replaces de + something, including every partitive and every quantity.
| Tu veux du pain ? | Oui, j'en veux. |
|---|---|
| Elle a des enfants ? | Elle en a trois. |
| Il revient de Paris. | Il en revient. |
| Tu as besoin d'aide ? | Oui, j'en ai besoin. |
With a number or a quantity, the number stays. This is the part learners drop:
- Combien de frères as-tu ? — J'en ai deux. (not j'ai deux)
- Vous voulez du café ? — Oui, j'en veux un peu.
- Il y en a beaucoup.
In English "I have two" is complete. In French j'ai deux is not a sentence — the en is compulsory, and leaving it out is the single most common error with this pronoun.
*Position is the same as y: before the verb, before the infinitive, after the verb in a positive command — Prends-en !*
*The two together, in this order: y then en. Il y en a.* — There is some / There are some of them.
The division of labour, in one line: y is à, en is de. Almost every use of both follows from which preposition the verb takes, so learning a verb with its preposition — penser à*, avoir besoin de* — is what makes the pronouns automatic.
Knowing the rule is not the same as producing it.
This page explains the pattern. The track drills it: the point arrives inside a mini-dialogue, then production marked against a CEFR descriptor, then a module test that returns you to your two weakest sessions if you miss it.