How do you know if a French word is masculine, feminine, and plura.?

Some adjectives, like "rouge", don't change from masculine to feminine.

With those that do, the feminine form often has an "e" on it. For instance green = vert (m) or verte (f).

The "s" is to make it plural.

If you're asking about nouns, there's no way to look at a word and know if it's masculine or feminine. There's no general rule.

''rouge'' is a color so it's an adjectif. You need to look at the noun to see if you must use an ''s'' or not. Such as ''les chiens rouges'' or ''la pomme rouge''. Rouge always take an e.