The "going to " Future

Many languages have a "future tense" to talk about the Future. English doesn't really have one. (Some books say "will" is the future tense but most grammar experts think that is not correct.) When we talk about the future, we use different forms to indicate what we already know or what we anticipate about the future.

When do we use "going to" ?

1. When we have already made some plan about what we are intending to do.
on Saturday morning as I'm really tired and then I'm going to go shopping for new clothes.

(Notice, however, that if we have already made an arrangement (e.g. bought the tickets) then we use the present continuous, not going to.
leaving?
holiday?
makes us sure about the future. going to rain.
I'm going to be sick.