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, devant, derrière, dans, etc. 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.  Sometimes y and en have no direct translation in English. 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.