C'est and il/elle est both translate to "he/she/it is" in English, but French picks between them based on what comes next, not on meaning. Get the pattern once and the choice becomes automatic.
1. Il/elle est + adjective, or a bare profession
Use il/elle est when what follows is just an adjective, or a profession or nationality with no article. See our être lesson for more on this.
| Il est grand. | He is tall. |
| Elle est intelligente. | She is smart. |
| Il est médecin. | He is a doctor. |
| Elle est française. | She is French. |
2. C'est + article/determiner + noun
Use c'est when a determiner (article, possessive, demonstrative) comes right before the noun.
| C'est un médecin. | He's a doctor. |
| C'est mon frère. | This is my brother. |
| C'est ce livre-là. | It's that book. |
| C'est intéressant. | It's interesting. (a general idea, not a specific person) |
3. The quick test
Ask yourself: is there a determiner right before the noun? An article (un, une, le, la...), a possessive (mon, son...), or a demonstrative (ce, cette...) means c'est. Just a bare adjective, or a bare profession/nationality, means il/elle est. Comparing the two: Il est médecin (profession alone) vs. C'est un médecin (article + noun), both correct, different structure, same meaning.
4. The plural: ce sont vs. ils/elles sont
| Ce sont mes parents. | These are my parents. (determiner + noun) |
| Ils sont sympas. | They are nice. (adjective alone) |
5. Quick reference
| il / elle est + adjective | Il est grand. |
| il / elle est + profession/nationality (no article) | Elle est française. |
| c'est + article/possessive/demonstrative + noun | C'est mon frère. |
| c'est + adjective (general idea) | C'est intéressant. |
| ce sont + plural noun with determiner | Ce sont mes amis. |