C'est masculin?

Question

This is one of the examples on the lesson: C'est une bonne nageuse. (She is a good swimmer.). I thought c'est had to be used with masculine only?

"When using il/elle, you have to make adjectives agree accordingly, whereas you always use the masculine with c'est."

Céline Pickard · Accepted Answer

Bonjour Chelsea,
"... whereas you always use the masculine with "c’est" " refers to cases where it is followed by a bare adjective (no noun):

C'est + [adjective]:

C'est beau ! = It is beautiful!
C'est bon ! = It is nice!
C'est dur. = It is hard.

In "c'est une bonne nageuse", "c'est" introduces a noun (i.e. "nageuse") which is feminine. The adjective is also referring to the word "nageuse". And therefore, it has to agree with the noun it refers to:

C'est + [noun]:

C'est ma sœur. = She is my sister.
C'est un livre. = It is a book.

C'est + [noun + adjective]:

C'est un bon élève. = He is a good student.
C'est un roman intéressant. = It is an interesting novel.
 
C'est une bonne idée. = It is a good idea.
C'est une grande maison. = It is a big house.

I hope this is helpful.
Bonne journée !