Three more future forms (+ 1)

the future progressive

There are three uses:

1. the future as 'a matter of course' 1
   The sentence
       He'll be working when you come
   implies that the action will be happening in the natural course of things and your arrival will interrupt it.  The form doesn't always denote a progressive action but it usually does.  The best way to explain this is probably with the use of a time line such as
   
2. the future as 'a matter of course' 2
   The second example
       I'll be starting work at 7
   does not imply a progressive action but the implication is that the action will be happening in the course of things and will mean that you are unavailable for other appointments.  It can also imply that this is a new arrangement rather than a one-off event so in that respect, it is progressive and similar to the first example.
3. tact and diplomacy
   the form is often used to sound tentative and polite.  Compare:
   

   Will you come at 6?	Will you be coming at 6?
   When will she install the new software?	When will she be installing the new software?

   The left-hand examples can sound peremptory and even rude but the use of the progressive form softens the nature of the question and makes it sound much more polite.
   This is somewhat subtle so you'll need to set the context and role relationships clearly if you are teaching the use.  It does, however, follow a general rule that more complex forms often imply more distance or politeness.

the future perfect

Use 1: He'll have finished the book by the time I want it

The tense is used to make it clear that one event or state precedes another.  As with the past perfect-past simple distinction, it is not always necessary to use the tense forms like this if the ordering is clear.  For example, it would be equally clear if one said
He'll finish the book and then he'll let me have it.

Use 2: He'll have repaired the car and then we can use it

This is clearly similar to Use 1 in the sense of ordering the activities but the distinction is that one event or action depends on another.  We can't use the car until it is repaired and the speaker makes this clear by the use of the perfect form.  It is not dissimilar to the use of the present perfect in, e.g.
When he has repaired the car, we can use it
except that the present perfect implies that he is working on the car now.  The future perfect can show that but it isn't necessary.

Use 3: I'll have been at the hotel for a day or two before I can call you

Another similar use to the first example but the speaker's implication here is a state will be in being before a subsequent event occurs.  This use is confined to verbs which do not normally occur in the progressive aspect like, be, have, like, understand etc.  See the guide to stative and dynamic verb uses for more.

Use 1: I will have been working for over two hours before you get here

The tense is also used to make it clear that one event or state precedes another.  The difference is simply that the speaker wishes to emphasise the duration of the action.  Compare, e.g.
I'll have been at work for over two hours before you get here
This is similar to Use 3 in the last list.  The speaker is making it clear that a progressive action (rather than a state of affairs like being in a hotel) will occur over an extended period before a subsequent (quick) event occurs.  It also implies that the progressive action may continue after the second event occurs.

Use 2: He'll have been travelling for ten hours and will be tired

This is clearly similar to Use 2 in the last list because one action or state causes or is dependent on the previous one.  Again, however, the speaker wishes to emphasise the duration of the action.