Rule 11.4: More Uses - the Future

1. To express a PRIOR PLAN only Be going to is used.

Examples Explanation
a) A: Why did you buy this paint?
B: I’m going to paint my bedroom tomorrow.

b) I talked to Bobby yesterday. He is tired of taking the metro to work. He’s going to buy a car. This is what he told me.

When the speaker is expressing a prior plan (something the speaker intends to do in the future because in the past s/he has made a plan or decision to do it), only be going to is used.*
In (a): Speaker B has made a prior plan. She decided to paint her bedroom last week. She intends to paint her bedroom tomorrow.
In (b): The speaker knows Bobby’s intention to buy a car. Bobby made the decision in the past and he intends to act on this decision in the future.
Will is not appropriate in (a) and (b).

2. To express WILLINGNESS only Will is used.

Examples Explanation

c) A: The doorbell’s ringing!
B: I’ll answer it!

d) A: I don’t understand this problem.
B: Ask your teacher about it. He’ll help you.

In (c): Speaker B is saying: I am willing, I am happy to answer the doorbell.” He is not making a prediction. He has made no prior plan to answer the doorbell. He is, instead, volunteering to answer the doorbell and uses will to show his willingness.
In (d): Speaker B feels sure about the teacher’s willingness to help.
Be going to is not appropriate in (c) and (d).

* Compare:

Situation 1
A: Are you busy this evening?
B: Yes, I’m going to meet Nathalie at the library at eight. We’re going to study together.

In situation 1, only be going to is possible. The speaker has a prior plan, so he uses be going to.

Situation 2
A: Are you busy this evening?
B: Well, I really haven’t made any plans. I’ll eat (or I’m going to eat) dinner, of course. Then I’ll probably watch (I’m probably going to watch) TV for a little while.

In situation 2, either will or be going to is possible. Speaker B has not planned his evening. He is “predicting” his evening (rather than stating any prior plans), so he may use either will or be going to.