BE + going to - Lindsay is going to fly to New York next week.
Forms with BE + going to possibly originated in such utterances as:
1. We are going to meet Andrea at the cinema,
uttered when we were literally going, i.e. on the way, to the meeting. At the moment of speaking there was present evidence of the future meeting. This use has become extended to embrace any action for which there is present evidence – things do not have to be literally moving. Consider now these two utterances:
2. Look at those black clouds. It's going to rain.
3. Luke is going to see Bob Dylan in concert next year
In [2] the present evidence is clear – the black clouds. In [3], the present evidence may be the tickets for the concert that the speaker has seen on Luke’s desk, or it may simply be the knowledge in the speaker's mind that s/he has somehow acquired.
Modal (will) - Lindsay will fly to New York next week.
Will is a modal and, like the other modals, has two core meanings. The two core meanings for most modal are:
(a) the 'extrinsic' meaning, referring to the probability of the event/state
(b) the 'intrinsic' meaning, reflecting such concepts as: ability, necessity,
obligation, necessity, permission, possibility, volition, etc.
The extrinsic meaning of will is exemplified in:
4. Emma left three hours ago, so she will be in Manchester by now.
5. There will be hotels on the moon within the next 50 years.
6. The afternoon will be bright and sunny, though there may be rain in the north.
In all three examples, the speaker suggests 100% probability, i.e. absolute certainty. (MAY would imply possibility, MUST logical certainty, to take examples of two other modals). Note that while certainty in [5] and [6] is about the future, in [4] it is about the present. It is the absolute certainty, in the minds of speaker/writer and listener/reader, that can give the impression that forms using ‘the will future’ are some way of presenting ‘the future as fact’. Some writers therefore call this form ‘the Future Simple’. Weather forecasters, writers of business/scientific reports, deliverers of presentations, etc, frequently use will, and learners who encounter English more through reading native writers than hearing native speakers informally may assume that it is a 'neutral' or 'formal' future. In fact the particular native writer or speaker is simply opting to stress certainty rather than arrangement, plan or present evidence.
The intrinsic meaning of will is exemplified in:
7. I'll carry your bag for you.
8. Will you drive me to the airport, please?
9. Jed will leave his mobile switched on in meetings. It's so annoying when it rings.
These examples show what we might loosely call volition, the willingness or determination of the subject of the modal to carry out the action. Note that [9] is not about the future, and in [7] and [8] the futurity is incidental. It is context rather than words which gives the meaning.
To understand this you need to firstly be aware of how to use the present continuous to talk about the future:
The present progressive indicating a future event speaks about arrangements for events at a time later than now. There is a suggestion that more than one person is aware of the event, and that some preparation has already happened. e.g.
- "I am playing with John tomorrow" is a one-time planned event (a single-time arrangement for the future).
The present simple is used when a future event is part of a programme or time-table.
The present simple is used to refer to events in the future which are certain because they are facts, or because there is a clear or fixed schedule or timetable.
- "I fly to Berlin next week." is a fixed event that is scheduled for next week.
Notice! Not every verb can be used to speak about the future (the sentence will sound awkward and generally unacceptable):
- My friend solves this problem tomorrow.
- My wife forgets about the party on Sunday.
A futurate is a sentence with unexpected future reference, and in which unplannable eventualities are generally unacceptable.
If we assume that generally a fact, an unchangeable programmed event, or a natural scheduled event can't be changed, the result, if it is, will be awkward. When laws of physics are questioned the sentence becomes really odd and generally unacceptable!
The earth is rotating slower tomorrow.The sun is setting slower tomorrow.
However, other futurate uses can only involve human agency or intention (thus the restriction). This mainly means that only due to human intervention a change is made or an event is to be:
- "The train is leaving at 8am tomorrow." (Normally it leaves at 8:20am but tomorrow due to changes made by human intervention the schedule is changed)
- "The shop is closing at 5pm on Friday." (Normally it stays open until 7pm but tomorrow due to changes made by human intervention is closes earlier)
To summarise, the proper "progressive futurate" is used with anything that can be changed by human intervention.
The meaning off the progressive futurate unlike that of the "progressive aspect" is not aspectual and thus doesn't show duration.
Sources:
Best Answer
There is a further use of "be going to": To talk events happening in the future as a consequence of the present state. "It is my birthday, so I'm going to be 30 tomorrow."
The pattern developed from a metaphor. We say "I'm going to London" to mean "there is a road which I will follow and at the end of the road is London". So when we use "going to" we get the idea of the future as being like places on a road.
So when we talk of prediction and intentions we mean "because I am on this 'road' I predict that this event will happen" or "because I choose to take this 'road' this event will happen".
If you say "I'm going to be 30 tomorrow", we are invoking this metaphor. "On the road of life, the place called "being 30" is tomorrow."
This metaphor is rich enough that nearly all future tense expressions can be expressed with "going to", or with "will". We tend to use "going to" with intentions or predictions, but we can use it for almost anything.