A1
c’est and il est
When French uses c’est rather than il est, the article rule that separates them, and how each behaves with adjectives.
Both translate as it is or he is, and they are not interchangeable. The thing that follows decides.
c’est before a noun with an article
C’est un bon livre.
it-is a good book
It’s a good book.
C’est mon frère.
it-is my brother
That’s my brother.
Anything with un, le, mon or a name in front of it takes c’est.
il est before an adjective or a bare profession
Il est fatigué.
he is tired
He’s tired.
Elle est avocate.
she is lawyer (f.)
She’s a lawyer.
C’est fatigué.
Il est fatigué.
An adjective describing someone takes il or elle. C’est fatigant — with the adjective meaning tiring — is a different sentence about a situation.
The same profession, two constructions
Il est médecin. C’est un bon médecin.
he is doctor. it-is a good doctor
He’s a doctor. He’s a good doctor.
The article is the whole difference. Bare noun after être takes il est; the moment an article appears it becomes c’est.
c’est for general statements
An adjective commenting on a situation rather than a person takes c’est, and the adjective stays masculine singular.
C’est difficile.
it-is difficult
It’s difficult.
C’est vrai ?
it-is true
Is that true?
The plural is ce sont
Ce sont mes amis.
these are my friends
These are my friends.
In speech c’est mes amis is extremely common and technically incorrect. Write ce sont.