Mastering the French pronouns y and en is essential to understand French conversation
Just try to follow an intermediate French class in Paris, and see how far you get!
En and y seem to be the ordinary French pronouns that are used to replace a noun in a phrase, but this is not the case
In reality, they replace words that denote quantities, qualities, properties and relations.
THE PRONOUN Y
Typically, the pronoun y replaces indirect objects following the preposition à (to), and it usually means “there” in English.
Example: Vous allez au cinéma ? (Are you going to the movie theater?).
Oui, je vais au cinéma. (Yes, I am going to the movie theater).
What we need is a pronoun to replace au cinéma. That pronoun is y.
Oui, j’y vais. (Yes, I’m going there).
Y can also replace à with non-person nouns that are not locations. When a verb takes an indirect object, you can replace that indirect object with the pronoun y.
Example: Tu penses à ta famille. (You’re thinking about your family.)
Tu y penses. (You’re thinking about it).
There are also many French expressions with y in them. These include, for example, il y a (there is) and allons-y (let’s go). These are fixed expressions, so you can take them at face value.
THE PRONOUN EN
In the same way that the relative pronoun y replaces constructions with à to give the meaning of “there”, the pronoun en replaces ones that use de (of) to give the meaning of “some” or “any”. Therefore, the pronoun en usually translates to “some” or “any” in English.
Example : J’ai beaucoup d’amis. (I have many friends).
J’en ai beaucoup. (I have many).
Nous voulons quatre croissants. (We want four croissants).
Nous en voulons quatre. (We want four of them).
It is important to note that the pronoun en is distinct from preposition en (in), as in “en France” or “en Espagne”.
There are also many French expressions with en in them, such as the ones below:
Exercises to master the difference between en and y
The following language websites will help you figure out in how to use y and en in French
About.com offers a good quiz to help you master the French pronouns.
Columbia University has one, too.