Hello, I am not clear where the article "the" must be used, may be used, must not be used. Please clarify.
We use it if there is only one of the entity we are thinking of, and would like the listener to think of it in that unique way as well.
In Indian English, we can use the bare noun without any article or number.
"I want to see movie tonight."
In English, we must put a/one/the as a specifier in front of the noun movie.
"I just want to see a movie tonight." (Any movie is good).
"I want to see the movie tonight." (This means a movie we have already agreed on, and are now thinking of in particular.)
"I want to see one movie tonight." (This is a bit less common but can be used if people are discussing the possibility of many movies or activities in one night).
There is only one President of the US, so when we refer to "US President" we use "the" to show we are thinkinig of a unique singular: "The US President will arrive in Dehli today."
Basically, we need some specifier before the important nouns, in international English, US English, and British English.