C'est v Il est

Question

You use "il/elle" for opinions and in specifying a particular item. But say "C'est une jolie robe."  I don't get it.

Accepted Answer

Bonjour Robin,
Indeed, it can be tricky to understand. So, in French, you have to show if the thing/item you are describing or talking about is either masculine or feminine. 
If an item/thing you are talking about has been previously mentioned, you will know if it is masculine or feminine. And as such, you will use il/elle est.
For example: Tu aimes mon pull? Oui, il est joli.
If an item/thing has not been previously mentioned, you will not know if it is masculine or feminine. And as such, you will use c'est + un/une/le/la/mon/ma...
For example: C'est une jolie robe! = That is a beautiful dress!
                    Qui est-ce? = who is it? -> C'est ma mère = It is my mother
I hope this is useful.
Bonne journée!

Answer

But even if it has been previously mentioned, and you do know the gender, if you want to describe something using a noun, you still have to use c'est + un/une/le/la/mon/ma...

You can't say il/elle est + un/une/le/la/mon/ma...

Just one of the idiosyncrasies of French that you have to remember, and surely the most frequently asked question on Kwiziq.