will, going to and shall

Will + infinitive and going to + infinitive are commonly used to talk about the future. Sometimes the difference between them is very small:
• John'U / John's going to meet us in the restaurant at 8 o'clock.
• Will you / Are you going to come back this evening?
However, going to is preferred in spoken English and will is preferred in formal written English.
In below, we focus on where there is a meaning difference.
we use going to rather than will when we PREDICT that something will happen in the future because we have some evidence for it now. It may be that we predict an event that is just about to happen on the basis of something that we feel, see (etc.) now:
• 'What's that matter with her?' 'She thinks she's going to faint.'
or it may be that we can predict an event because we have been told that it will happen:
• Did you know that Bob and Kath are going to get married?
However, if we make a prediction based on our opinion or our past experience we use will:
• Why not come over at the weekend? The children will enjoy seeing you again.
• I imagine the stadium will be full for the match on Saturday.
When we talk about INTENTIONS or DECISIONS about the future that were made some time before we report them, we prefer going to or the present continuous
• 'Who's arranging the party?' 'Jo's going to do it.' (- this has been planned)
• Toni told me that she's going to move back to Spain. (= reporting an intention)
However, notice that in a formal style, we use will rather than going to to talk about future events that have been previously arranged in some detail:
• The meeting will begin at 10.00 am. Coffee will be available from 9.30 onwards.
When we state a decision made at the moment of speaking, we prefer will:
• 'Is that the phone?"Don't worry. I'll get it.'
• It's late. I think I'll go to bed now.
We can use will or 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 - it will happen if something else happens first:
• If we go on like this, we'll / we're going to lose all our money.
• You'll / You're going to knock that glass over (if you're not more careful).
However, we use will (or another auxiliary), not going to, when we describe a future event that follows another. Often 'if' has a meaning similar to 'when' in this kind of sentence:
• If you look carefully, you'll (or can) find writing scratched on the glass.
• If you move to your left, you'll (or may) be able to see the church.
We can use shall (or shan't) instead of will (or won't) in statements about the future with I and we, although it is more common to use will/won't:
• When I retire, / shall/will have more time for my painting.
• The stronger we are, the more we shall/will be able to help others.
• In current English we don't use shall/shan't with other subjects (it, she, they, etc.) when we talk ' about the future.