1. “Will” is used to make prediction. There is no evidence. It is just our personal opinion.
- Perhaps it will rain. (It is just prediction. )
- Maybe he will come again. (It is just prediction. )
“ Be Going To” is used to make predicition. If there is an evidence, it is preferred instead of “will”.
- Look at those black clouds. It is going to rain. ( There is an evidence, most probably being rainly )
- Be careful! The bee is going to bite you.
2. “Will” is used for unplanned actions in future.
- I will fly to England. ( I don’t have a flight ticket or reservation. )
- We will see you again. ( just We think to see you again in future, there is no a definite or exact date or plan )
- She will be a doctor. ( She is not a university students but she wants to be a doctor in future )
“ Be Going To” is used to indicate planned actions.
- I have got a flight ticket to 3 o’clock tomorrow. I‘m going to fly to England.
- They have made reservations for their summer holiday. They’re going to spend their holiday in London.
- Jully is calling her friends for breakfast. She is going to make a delicious breakfast tomorrow.