The choice between c’est (and its plural ce sont) and il est (and its feminine elle est, plural ils sont and elles sont) is a basic but important part of french grammar. They both translate as “it is,” “he is,” or “she is,” but they are not used in the same way.

The general rule

Use c’est to introduce or identify something. It points to a thing or a person. It is followed by a noun, or a word that acts like a noun.

Use il est (or elle est) to describe something. It gives information about a specific thing or person already known. It is followed by an adjective, or a word that acts like an adjective.

C’est + noun (or noun equivalent)

This structure is for identification. It answers the question “What is it?” or “Who is it?” The noun is often modified by an article, a number, or a possessive word.

• C’est un homme.
  “He is a man.” (Identification: he belongs to the group of men.)
• C’est un docteur.
  “He is a doctor.” (Identification: his profession.)
• C’est la maison de mon enfance.
  “It is the house of my childhood.” (Identification: which house.)
• Ce sont mes amis.
  “They are my friends.” (Identification: who they are.)
• C’est difficile, la physique.
  “Physics is difficult.” (Here c’est introduces the idea that follows.)

If you tried to say Il est un homme, it would be incorrect. It sounds like “He is a man” in english, but in french it is not used.

Il est + adjective (or adjective equivalent)

This structure is for description. It tells you something about a specific person or thing that has already been mentioned or is obvious from context.

• Regarde cet homme. Il est grand.
  “Look at that man. He is tall.” (Description of the man.)
• J’aime cette table. Elle est solide.
  “I like this table. It is sturdy.” (Description of the table.)
• Ils sont contents.
  “They are happy.” (Description of their state.)
• Il est interdit de fumer ici.
  “It is forbidden to smoke here.” (Description of the action.)

Special cases and exceptions

There are some common situations that seem to break the rule, but they have their own logic.

1. Professions without an article
When stating someone’s profession without any modifier (like an adjective), you use il est + profession. This is a description of their job, not an identification.

• Il est docteur.
  “He is a doctor.” (Describing his job.)
• Elle est professeur.
  “She is a teacher.” (Describing her job.)

Compare this with C’est un bon docteur. Here you add an adjective (bon), so the noun now needs an article, and you switch to c’est.

2. Religion and nationality
The same rule applies to religion and nationality when used without an article.

• Il est catholique.
  “He is catholic.”
• Elle est française.
  “She is french.”

3. Il est with time
To tell the time, you always use il est.

• Il est trois heures.
  “It is three o’clock.”

4. C’est with a stressed pronoun
To emphasize a person, you use c’est followed by a stressed pronoun (moi, toi, lui, elle, nous, vous, eux, elles).

• C’est moi.
  “It’s me.”
• C’est elle qui a préparé le dîner.
  “It’s she who prepared dinner.”

5. C’est + adjective + de + infinitive
When an adjective describes an action (introduced by de), you use c’est.

• C’est important de manger sainement.
  “It’s important to eat healthily.”
• C’est gentil de votre part.
  “That’s kind of you.”

Synonyms and alternatives

There is no direct synonym for the grammatical distinction itself. The choice is forced by the structure. However, you can sometimes rephrase.

• Instead of Il est grand, you could say C’est un homme grand.
• Instead of C’est un docteur, you could say Il est docteur (if you mean his job, not introducing him).

Summary

• Use c’est + noun (with article) to identify or introduce.
• Use il est + adjective to describe.
• Use il est + profession/nationality (without article) to state someone’s job or origin.
• Use il est for time.