"Il est un [noun]" is a no-go.

Whenever "il/elle est" is followed by a modified noun, you have to change the pronoun and use "c'est + modifier + noun".

The same applies to the plural "ils/elles sont" to be replaced with "ce sont".

Modifiers can be articles, demonstrative adjectives, possessive adjectives, indefinitive adjectives or numbers:

• she is a woman = c'est une femme
• they are good citizens = ce sont de bons citoyens/bonnes citoyennes
• he is my brother = c'est mon frère
• they are two large dogs = ce sont deux grands chiens