Will and Will Be Going To

A. We can use either 'will' or be 'going to' to talk about something that is planned, or something that we think is likely to happen in the future:

1. We will study climate change in a later part of the course. (or We are going to study...)
2. Where will you stay in Berlin? (or Where are you going to stay...?)
3. The south of the city won't be affected by the power cuts. (or ...isn't going to be affected...)

We often prefer be going to in informal contexts

B. We use will rather than be going to to make a prediction based on our opinion or experience:

1. Why not come over at the weekend? The children will enjoy seeing you again.
2. 'Shall I ask Sandra? No, she won't want to be disturbed.'

We use be going to rather than will when we make a prediction based on some present evidence:

1. The sky has gone really dark. There's going to be a storm.
2. 'What's the matter with her?' 'It looks like she's going to faint.'

C. We use will rather than be going to to make a prediction based on our opinion or experience:

1. Why not come over at the weekend? The children will enjoy seeing you again.
2. 'Shall I ask Sandra? No, she won't want to be disturbed.'

We use be going to rather than will when we make a prediction based on some present evidence:

1. The sky has gone really dark. There's going to be a storm.
2. 'What's the matter with her?' 'It looks like she's going to faint.'

D. To predict the future, we often use will with I bet (informal), I expect, I hope, I imagine, I reckon (informal), I think, I wonder, and I'm sure, and in questions with think and reckon:

1. I imagine the stadium will be full for the match on Saturday.
2. That cheese smells awful. I bet nobody will eat it.
3. When do you think you'll finish work?
4. Do you reckon he'll say yes?

Be going to can also be used with these phrases, particularly in informal contexts.

E. We use will when we make a decision at the moment of speaking and be going to for decisions about the future that have already been made

Compare:

1. I'll pick him up at 8.00. (an offer; making an arrangement now)
2. I'm going to collect the children at 8.00. (this was previously arranged)
3. 'Pineapples are on special offer this week.' In that case, I'll buy two.'
4. When I've saved up enough money, I'm going to buy a digital camera.]

However, in a formal style, we use will rather than be going to to talk about future events that have been previously arranged in some detail.

Compare:

"Are you going to talk at the meeting tonight?"
And
"The meeting will begin at 9.00 a.m. Refreshments will be available from 8.30 onwards." 

F. We can use will or be going to with little difference in meaning in the main clause of an if-sentence when we say that something (often something negative) is conditional on something else:

1. If we go on like this, we'll/we're going to lose all our money.
2. You'll/You're going to knock that glass over if you're not more careful.

When the future event does not depend on the action described in the if-clause, we use be going to, not will. This kind of sentence is mainly found in spoken English.

Compare:
I'm going to open a bottle of lemonade, if you want some.=( I'm going to open a bottle of lemonade. Do you want some?)
And
I'll open a bottle of lemonade if you want some.=( If you say you want some, I'll open a bottle.)

Try this exercise to test your grammar.

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