Learn English – Does “I will” express a current desire, or a prediction of the future

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.

There is no such way of casting any of the modal verbs can/could, may/might, must, shall/should, will/would in future form with will.

This is because the modal verbs are all defective: they have only two forms, past and present (must has only one), so they lack the infinitive form which follows auxiliary will.

There are two ways around this.

1. The simple present of most verbs can be used with future reference, so one thing you can do is simply use can with some indication of futurity.

   I cannot do that tomorrow.
   I may do that tomorrow.

2. The other thing you can do is employ the infinitive of a "periphrastic" construction as the complement of will. With can, for instance, the periphrastic construction is BE able to; with will it's BE going to.

   I will not be able to do that.
   I will be going to do that.

   Other periphrastics which come in handy for this are:

   for may/might: be permitted/allowed to: He will be permitted to do that.
   for must: be obliged/required to: He will be obliged to do that.
   for should: be expected to: He will be expected to do that.
   for will: be going to: He will be going to do that.

   There are more, because all the modals have a wide variety of meanings, and many of these meanings have one or more periphrastics.

Note that the modal verbs have no participles either, so they can't be cast in the perfect construction or employed as gerunds or adjectives. Again, the way around this is to employ the appropriate periphrastic:

I have been able to do that.
Being able to do that would be helpful.

Modal verbs are intransitive and can't be cast in the passive, and they're stative and therefore can't be cast in the progressive, so you don't have to worry about those forms.