The pronouns Y and EN

Y means usually means there and sometimes it. It replaces prepositional phrases beginning with à (or any contraction of it), en, sur, sous, chez, or dans.

En means some or some (of them), or of it. It replaces prepositional phrases beginning with de (or any contraction of it) or a number

EN cannot replace people unless the person is introduced with an indefinite article, partitive, number or quantity. Remember that they go before the verb, except in a command, in which they follow the verb and are connected with a hyphen.
The -er verbs also add the -s they lost when forming the you (familiar) command.

Examples

Do you want some ? Voulez-vous des pommes?
Do you want some? Vous en voulez?
I have three sisters. J'ai trois sours.
I have three (of them). J'en ai trois.
It is in the drawer Il est dans le tiroir.
It is there. Il y est.
I am going to Europe Je vais en Europe
I am going there. J'y vais.
I am going to go to Rome. Je vais aller à Rome.
I am going to go there. Je vais y aller.
I am coming back from London Je reviens de Londres.
I am coming back from there. J'en reviens
Answer the telephone! Répondez au téléphone!
Answer it! (formal) Répondez-y!
Stay there! (familiar) Restes-y!
Don't stay there! (familiar) N'y reste pas.