Differences Between Future Tenses

1. We use "going to" to talk about a planned activity for future. 

For instance:

A: There is a good movie on TV tonight.
B: Yes. I know. I am going to watch it.

A: Why is Betty in a hurry?
B: Because she is going to meet her brother at the station at six.


2. We use "will" when we decide to do something at the time of speaking.

For instance :

A: What would you like to drink, sir?
B: I will have a glass of milk, please.


A: Oh, you have left the door open.
B: Have I? I will go and shut it.


3. We use "going to" for prediction in the near future. The speaker is sure because there are signs about it. 

For instance :

She is standing at the edge of the cliff, she is going to fall.

A: What is the problem? There are strange noises in the lift.
B: I think the lift is going to break down .Let’s get out!

A: I feel cold.
B: Look at those black clouds. It is going to rain.

A: What is the matter with you?
B: I feel terrible. I think I am going to be ill.


4. We use "will" for prediction in the remote future 

For instance :

A: How is he, doctor?
B: Don’t worry. He will get better.

A: The bridge is not so safe.
B: Yes. One day it will break down.


5. We use "will" for requests, invitations and offers.

For instance :

Will you shut the door, please? (Request)
Will you come to the cinema with me? (Invitation)
That bag looks very heavy. I will help you. (Offer)


6. We generally use "will" for prediction with the following verbs and phrases.

Believe , expect , hope , think , wonder , I’m sure , I’m afraid ...etc

I believe he will pass the exam. 
I hope he won’t leave us.
I am afraid he will fail the exam.
Do you think they will divorce?


7. We use "will" with the adverbs Perhaps, probably, certainly

We do not use going to in this case.

Perhaps they will support you in the election.
Jackson will probably go to London next year.


8. We use "will" to express promise, not going to 

I will study medicine at university. 
I will stop going to the casino.


9. We use "will" in a question tag after imperative, not going to.

Do it today, will you?
Don’t look at your friend’s paper, will you?


10. We use "will" to give an order or state our wishes, not be going to.

Will you turn on the lights?


11. We use "will" to express an action in necessity in the future, not be going to. 

You will have to pay a fine if you don’t pay your tax on time.


12. We use "will" for threat. 

Study hard or you will fail.
Give me all your money or you will die.