I guess I can't fully understand a difference between "en" and COD in this part

Question

As I know we use "en" to replace a noun that follows number/quantity such as un(e), deux, un peu de etc.. so in this case it means that I can use "en" to replace ANY noun including "idée, histoire, conseil, chance" etc but only with any indefinite article, right? And in examples like "je vais vous donner une idee" "il raconte une histoire à mon ami" these nouns can be also replaced with "en", right? or not? why? I passed several tests where these nouns were replaced with COD(le,la,les) and I really can't understand why.. can someone explain it to me, please?

Accepted Answer

Hi Lenni,
Below is a sample of the Kwiziq lessons you need to study -
En with quantities = Of them (French Adverbial Pronouns)
Which tests are you referring to? Unless they are our tests we cannot comment.
Hope this helps!

Answer

Definitely go through the lesson Cécile linked to in her answer. Here are a few examples that may shed a bit of light on the issue.

In the following three sentences une histoire is the COD. It is replaced by the corresponding pronoun (la). And then we're using en to refer to a specific story among a group of several.

Il me raconte une histoire. -- He tells me a/one story.
Il me la raconte. -- He tells me it.
Il m'en raconte une. -- He tells me one (of several stories).

In the next example, l'histoire is the indirect object introduced by the preposition de. This kind of COI is replaced by en in the next sentence.

Il me raconte de l'histoire. -- He tells me about the story.
Il m'en raconte. -- He tells me about it.