We use the special going to construction when we have the intention to do something before we speak. We have already made a decision before speaking. Look at these examples:
- I have won $1,000. I am going to buy a new TV.
- We're not going to see my mother tomorrow.
- When are you going to go on holiday?
In these examples, we had an intention or plan before speaking. The decision was made before we spoke.
We often use going to to make a prediction about the future. Our prediction is based on evidence. We are saying what seems sure to happen. Here are some examples:
- The sky is very black. It is going to snow.
- It's 8.30! You're going to miss the train!
- I crashed the company car. My boss isn't going to be very happy!
In these examples, the present situation (black sky/the time/damaged car) gives us a good idea of what is going to happen.
- We use will for prediction when we have no real evidence: "It will rain tomorrow." (It's my feeling but I can't be sure.)
- We use going to for prediction when there is some real evidence: "It's going to rain." (There's a big, black cloud in the sky and if it doesn't rain I'll be very surprised.)