Will
Use will + infinitive
- To make predictions or talk about future facts
It looks as if the economy will slow down next year.
- To make decisions at the time of speaking
A: I’m sorry – I’m really busy at the moment.
B: Don’t worry – I’ll call back later.
For decisions made earlier, when you mean I’ve decided to, use going to, not will.
I should be back in an hour. I’m going to get my hair cut. (NOT: I will get…)
Be going to
Use be going to + infinitive
- To talk about a plan or intention where the decision has already been made
A: I’ve asked the contractors to meet with us.
B: I see. What are you going to say to them?
- To make a very definite prediction based on evidence that you can see or know about.
My manager likes to start meetings on time, so he’s going to be annoyed when I arrive late.
Often either will or going to can be used to make predictions.
When interest rates go up, people will/are going to start spending less.
Present continuous
Use the present continuous to talk about arrangements, appointments, social events, and anything you would put in a diary, particularly when the time, place, or purpose is mentioned.
I’m seeing Bill in Paris tomorrow to discuss the project.
Going to can almost always be used in these situations, but use of the present continuous is very common in everyday spoken English.
Present simple
Use the present simple to refer to future events that are in a timetable.
Do you know when the last train leaves?
The verb be is often used in the present simple when talking about personal schedules.
I’m in Madrid on Friday and I’m away for a couple of days next week as well.