En is a pronoun that typically replaces de + a noun; this includes nouns introduced by partitive or indefinite determiners (de, du, de l', de la, des).
En may be translated as 'some', 'any', or 'not any'.
En replaces a noun introduced by a number or an expression of quantity. Notice that the equivalent of en is not always expressed in English, but en must still be used in the French sentence:
En also replaces expressions introduced by the preposition de with the following verbs:
s'occuper de, to deal with
parler de, to speak of
remercier de, to thank for
revenir de, to return from
venir de, to come from
NOTE: a disjunctive pronoun (lui, leur) is used with these verbs to replace expressions when the object of the preposition de is a person rather than a thing. (J’ai besoin de mes parents. J’ai besoin d’eux).
Placement of en is the same as direct and indirect pronoun objects. En comes before the verb it refers to, except in the affirmative imperative (Parles-en! Talk about it)..
In compound tenses (such as the futur proche and passé composé), it precedes/comes before the auxiliary verb.
NOTE: There is no agreement between en and the past participle. Il a lu deux livres. Il en a lu deux. He read two books. He read two of them.
When en is used with il y a ('there is, there are'), it comes between y and a: Du pain sur la table? Il y en a (Some bread on the table? There is some there.)