'be going to'?

We use 'will' when we make a decision about

the future at the moment of speaking.

'What do I want to drink? I think I will have a coffee.'

We use 'be going to' when the decision is made

before speaking.

'I know what I want!

I am going to have a tea.'

We can also use 'will' to talk about predictions

based on something we expect to happen.

'I will probably be late because

the buses are often delayed.'

We use 'be going to to' talk about a predictions

based on evidence.

'Look at those black clouds! It is going to rain soon.'

Right I'm off now!

I'm going to buy a coffee.

Bye!