Page 224 - Practical English Usage 3ed - Michael Swan, Oxford

future (7): will and shall (interpersonal uses) 217
- Look out - we're going to crash! (There is outside evidence.)
Don't lend him your car. He's a terrible driver - he'll crash it. (the speaker's
knowledge)
- I've just heard from the builder. That roof repair's going to cost £7,000.
(outside evidence - the builder's letter)
I reckon it'll cost about £3,000 to put in new lights. (the speaker's opinion)
- Alice is going to have a baby. (outside evidence - she is pregnant now) The baby will certainly have blue eyes, because both parents have. (speaker's
knowledge about genetics)
217 future (7): will and shall (interpersonal uses)
1 differences between will and shall
Will and shall are not only used for giving information about the future. They are also common in offers, promises, orders and similar kinds of 'interpersonal' language use. In these cases, will (or 'll) generally expresses willingness or wishes (this is connected with an older use of will to mean 'wish' or 'want'). Shall expresses obligation (like a more direct form of should).
2 announcing decisions: will
We often use will when we tell people about a decision as we make it, for instance if we are agreeing to do something.
OK. We'll buy the tickets. You can buy supper after the show.
The phone's ringing. -I'll answer it. (NOT I'm gBing tB answet' it.)
Remember to phone Joe, won't you?- Yes, I will. Shall is not used in this way.
You can have itfor £50. - OK. I'll buy it. (NOT ••• ( shaH buy it.)
Note that the simple present is not normally used to announce decisions.
I think I'll go to bed. (NOT .. think" gB tB bed.)
There's the doorbell. - I'll go. (NOT •• •-f-gtt:)
To announce decisions that have already been made, we generally prefer going
to . .. or the present progressive (see 213-214).
Well, we've agreed on a price, and I'm going to buy it. I've made my decision and I'm sticking to it.
Stressed will can express determination. I will stop smoking! I really will!
3 promises and threats: will
We often use will/'ll in promises and threats. Note that the simple present is not possible in these cases.
I promise I won't smoke again. (NOT : promise: Mn't smoke ...) I'll phone you tonight. (NOT : phlme ...)
I'll hit you ifyou do that again. You'll SUffer for this!
Shall is also possible in British English after I and we, but it is less common than will.
I shall give you a teddy bear for your birthday.
In older English, shall was often used with second and third person subjects in promises and threats. This is now very unusual.
You shall have all you wish for. He shall regret this.
page 192