has a function as pronoun and en
has also a function as partitive article.
a) Est-ce que tu te souviens de son nom? Oui, je m' en souviens encore.
Do you remember his name? Yes, I still remember it.
b) Est-que tu lui as donné de la farine? Oui, je lui en ai donné.
Did you give him flour? Yes, I gave him some of it.
c) Est-qu' il connait les détails de l' histoire? Oui, il en connait les détails.
Does he know the details of the story? Yes, he knows details of it.
In clause a) en
represents all of the noun (it represents the name). In b) and c) en represents only part of the substitute. However, even if we can make this difference, the French do not differentiate between the full context or noun and part of it.
The position of en is after the pronouns, but before the first conjugated verb or infinitive.