be going to is used

for plans we have for the future.

I'm going to plant a tree in spring.

I'm not going to have breakfast. I'm not hungry.

Are you going to eat it? It looks delicious.

be going to is used

in predictions when we can see that something will soon happen.

Look at him! He's going to win the sack race.

Oh dear! It's 8 o'clock and I'm not ready yet.

I'm going to miss the train.

The future simple is used

• in predictions about the future. People will live longer in the future.

• with the verbs believe, think etc. I think she'll pass the exam.

• with the phrases be sure, be afraid, etc. I'm sure he'll be late again.

• with the adverbs certainly, perhaps, probably, maybe, etc. We'll probably eat out this evening.

The future simple is used

• for decisions we make at the moment of speaking. I like this dress. I will buy it.

• for offers. Since you're tired, Mum, I will do the shopping today.

• for events and situations which will definitely happen in the future and which we cannot control. My little sister will be three years old next month.

• for promises, warnings, hopes. I promise that I'll be home for dinner.