What do y and en actually replace in French?
Y replaces a phrase beginning with à — a place, or a thing introduced by à. En replaces a phrase beginning with de, and also any quantity: some of it, three of them, none. Both go directly before the verb, and both are compulsory where English would simply drop the object.
Y replaces à plus something
Most often that is a location, which is why courses introduce it as there. But it equally replaces à plus a thing after verbs like penser à and répondre à. It does not replace à plus a person — that takes an indirect object pronoun instead.
Je vais à Paris. → J'y vais.
I'm going to Paris. → I'm going there.
Je pense à mon travail. → J'y pense.
I'm thinking about my work. → I'm thinking about it.
Je pense à Marie. → Je pense à elle.
A person keeps the stressed pronoun, not y.
En replaces de plus something, and quantities
En covers de plus a noun after verbs like parler de and avoir besoin de, and it also stands in for any quantity. This second use is where English speakers most often omit it, because English lets you say I have three and French requires j'en ai trois.
Je parle de mon voyage. → J'en parle.
I'm talking about my trip. → I'm talking about it.
Tu as des pommes ? → Oui, j'en ai.
Do you have apples? → Yes, I have some.
J'en ai trois.
I have three (of them).
En is compulsory even with the number stated.
Fixed expressions
Several very common phrases contain y or en with no recoverable antecedent at all. They are idioms and are best learned whole rather than analysed.
Il y a
There is / there are
Ça y est
That's it / it's done
Je m'en vais
I'm leaving
Il y en a
There are some (of them)
Common mistakes
J'ai trois.
J'en ai trois.
French cannot leave the quantity hanging. En is required even when the number is stated.
Je pense à elle → J'y pense.
Je pense à elle.
Y does not replace people. À plus a person takes a stressed or indirect pronoun.
Je vais y aller à Paris.
Je vais y aller. / Je vais aller à Paris.
Y already contains à Paris, so stating both is a duplication.
Frequently asked questions
How do I remember which is which?
Match the preposition. Y stands in for à, en stands in for de. Since most verbs govern one preposition or the other, learning verbs with their preposition attached — penser à, parler de — tells you the pronoun automatically.
Where do they go in the sentence?
Directly before the conjugated verb, or before the infinitive in a two-verb construction: j'y vais, je veux y aller. In the passé composé they precede the auxiliary — j'y suis allé.
What if both appear together?
En comes last: il y en a. The order of French object pronouns is fixed and y precedes en in every combination where they meet.