Y and en are both pronouns that go before the verb.  Y (ee) means it or there.  En (awn) means some or some (of them), or of it.  They replace prepositional phrases.  In French, the phrases will begin with à (or any contraction of it), en, sur, sous, chez, or dans for y; and de (or any contraction of it) or a number for en.  They 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.

Note:  Notice y and en don't go after the verb in negative commands. Treat them like pronouns.   Ne or Je plus y or en all contract to N'y (nee), J'y (zhee), N'en (nawn), and J'en (zhawn).  When you have a conjugated verb plus an infinitive (vais and aller), the y or en go in between the two verbs.