What is the difference between 'y' and 'en'?

The simplest way to think of these prepositions is to think that 'y' refers to 'à' and 'en' refers to 'de'. Therefore if you want to say 'Je vais à Paris' you would then say 'J'y vais'.

If you want to use 'en' with the sentence 'Je mange du gâteau', this becomes 'J'en mange'.

Therefore, 'y' refers to a place or a verb followed by 'à' and 'en' refers to a quantity or a verb followed by 'de'. In terms of sequence of prepositions, 'y' comes before 'en'.