RULES OF THUMB

Will vs Going To

Option 1

Ignore it completely, especially at lower levels.

The difference between will and going to (not to mention other ways of expressing futurity) is so subtle that it cannot be easily conveyed through rules or isolated examples. Moreover, often the two forms are interchangeable (I think it will rain/I think it’s going to rain) or the meaning overlaps to such an extent that there is no risk of the learner being misunderstood.

Option 2

Teach some 'rules of thumb', and hope that these are sufficient to deal with the majority of instances that learners will come across. For example:

'Going to' is a kind of present tense – look at its form! – so you use it when you want to talk about a future situation that is already connected to the present, e.g. because there’s present evidence, or because a plan is already in motion:

e.g. - I think it’s going to rain – I just felt a drop.
They’re going to retire to the country – they’ve already bought a little cottage.

In other cases, where there is no implicit or explicit connection to the present, use will.

e.g. - The concert will be over by midnight. I’ll light the barbecue.