C'est or il est
CEFR A1 · part 2
edumaterial's parts within the level. The European framework does not subdivide them.
The partitive: du, de la, desUsing du, de la or des before uncountable things, and switching to de after a negative.à and de with the article: au, aux, du, desContracting à + le into au, à + les into aux, de + le into du and de + les into des…Adjectives: before or after the nounPlacing the adjective correctly and making it agree, including irregular feminine forms.Passé composé: avoir or être?Choosing the auxiliary, forming the participle and agreeing when required.Son, sa, ses: they agree with the thingChoosing the possessive by the gender and number of the thing owned, not the owner; and…The three ways of asking a questionAsking the same question three ways and knowing which each situation calls for…The rule, in three lines
C'est is followed by something: an article, a proper name, a pronoun — c'est un médecin, c'est Marie. Il/elle est takes a job or nationality with no article: il est médecin. And in the plural, ce sont.
The usual slip. Putting an article after il est: «il est un médecin». With a bare job there is no article; with an article you need c'est.
Why it catches you out. English always puts an article —he is a doctor— so you put one in French too, and il est un médecin is exactly what comes out. Yours is the only one of the four that has to LOSE something here.