When there are definite signs that something is going to happen. (Evidence)

Something is likely to happen based on the evidence or experience you have.

• I think it is going to rain - I just felt a drop.
• I don't feel well. I think I 'm going to throw up. (throw up = vomit)

3. When something is about to happen:

• Get back! The bomb is going to explode.

When to use WILL

In other cases, where there is no implicit or explicit connection to the present, use WILL:

For things that we decide to do now. (Rapid Decisions)

This is when you make a decision at that moment, in a spontaneous way.

• I'll buy one for you too.
• I think I'll try one of those. (I just decided this right now)

When we think or believe something about the future. (Prediction)

• My team will not win the league this season.
• I think it will rain later so take an umbrella with you.

Note: You can use both Will and Going to for making future predictions.

To make an offer, a promise or a threat.

• I'll give you a discount if you buy it right now.
• I promise I will behave next time.
• I'll take you to the movies if you'd like.

You use WON'T when someone refuses to do something.

• I told him to take out the trash but he won't do it.
• My kids won't listen to anything I say.
• My car won't start.

Future Predictions

As you can see, both Will and Going to can be used for making future predictions without having a real difference in meaning.

• The weather report says it will rain tomorrow. (Correct)
• The weather report says it is going to rain tomorrow. (Correct)