what if both a and de are there in the sentence what am i supposed to use en or y ?

Bonjour Susan ! You will never use both "y" and "en" together in the same sentence. In such a case, you will use either "en" or "y" to replace one of the groups, and keep the original other group. "Je veux des bonbons à la plage." (I want sweets at the beach.) -> "J'y veux des bonbons." (Not my favourite option, it doesn't sound great to my French ears!) -> "J'en veux à la plage." I hope that's helpful! À bientôt !

"You will never use both "y" and "en" together in the same sentence." - I often hear the phrase "Y en a." in response to a question like "Il y a des pommes dans ce magasin?", for example. Is it then wrong to use "y" and "en" together in the above sentence?