It is very important to know whether the noun you’re talking about is masculine or feminine. This is because others words that we use to do with the thing we’re talking about need to agree with our noun.
For example, if we’re talking about ‘the black chair’, the word for chair is ‘silla’. This ends in ‘a’ so it’s feminine hence the version of ‘the’ and the version of ‘black’ we’re using also need to be feminine.
We have to say ‘La silla negra’.
It wouldn’t make sense to use ‘negro’ (the masculine version of ‘black’ as we are talking about the chair which is feminine so it has to agree).
The way I think of this is like a matching up game, if the noun ends in ‘a’ our adjective (in this case ‘black-negra’) has to end in an ‘a’ too!
