This is the weather forecast. Why do you use it?

Theweather in this sentence refers to the local real-time weather, that is, the weather here and now. This is a special reference, so the article The must be added. Articles indicate the nature and quantity of nouns, and whether nouns refer generally or specifically. Articles include definite articles and indefinite articles. The expression of indefinite article corresponding to Chinese is one and some, and definite article corresponding to this and these. Usage is the same as English articles.

The definite article is a function word, which cannot be used alone and has no meaning. Used before nouns to help express the meaning of nouns. Different from indefinite articles, definite articles have definite meanings and are used to refer to people or things, indicating that the person or thing referred to by nouns is a specific one of the same kind, which is different from other people or things of the same kind and equivalent to that or this in Chinese. Can be used with singular and plural nouns or uncountable nouns.