Will vs Be Going To
BE GOING TO – It is clear that the speaker has already given the action a lot of previous thought and planning.
- Yasmin bought a lot of food this morning and borrowed my DVD player. She’s going to have a party tonight.
- MOTHER: What are you doing with that book?
LITTLE BOY: I’m going to study before dinner.
- We bought a new car last week, and we got some maps. We’re going to drive to Banff next month.
WILL – It is clear that the speaker has not given the action previous thought or planning and that they are simply expressing a willingness to do something..
- TEACHER: I left the grammar tests in my office.
STUDENT: I’ll get them for you.
- MOOHYUN: The phone’s ringing.
MIJOO: Don’t get up. I’ll get it.
Sometimes it is not clear (we don’t know) whether or not the speaker has given the action previous thought or planning. The speaker may just be making a prediction about something that might happen. Both can be used.
- My parents will arrive (are going to arrive) at 8:00 tonight.
- It will probably rain (is probably going to rain) this evening.
- We will need (are going to need) some new books next semester.