"My book is black."
Translation:Mon livre est noir.
Ikkiqyy, "noir" is used when describing masculine nouns (e.g. Le vin est noir) whereas "noire" is used when describing feminine nouns (e.g. La pomme est noire).
Assuming you are referring to using c'est instead of est in this sentence it is because c'est translates as this is whereas what the sentence requires is simply is/est.
Just a few days ago the exact sentence had "noire" for black. Why the difference?
When the noun that noir modifies is masculine as in le livre then the adjective takes the masculine form noir . If the phrase was a black woman then it would be une femme noire because the adjective must agree with the noun. Une agrees with femme and so does noire.
The verb to be ... être
Je suis....I am
Tu es.......You are (sing.)
Il,elle, est...He, She is
Nous sommes..We are
Vous êtes ......You are (pl) (formal sing.)
Ils, Elles, sont..They are
Livre is masculine therefore mon (masc.) is used instead of ma (fem).
Mon is substituted for ma only when ma is placed in front of a word starting with a vowel. Other than that mon/ma must agree with the noun they modify.
Hi Melsin, in general, we use "mon" to describe nouns that are masculine (e.g. Mon livre est bon) unless in cases when that noun begins with a vowel (e.g. Ma animal est bon). We always use "Ma" to describe nouns that are feminine (e.g. Ma pomme est bonne).