Three more future forms (+ 1)

The future progressive

There are four uses:

1. the future as 'a matter of course' 1
   The sentence
       They will be having dinner so don't call them now
   implies that the action will be happening in the natural course of things and your call would 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
   
   This form is also used to refer to a continuous background state which may not actually be an event in progress at the time to which the speaker refers.
   For example:
       They'll be staying in a hotel for a while
   refers not to where the people in question currently are but to a background continuous situation ongoing from the present.
2. the future as 'a matter of course' 2
   The second example
       She will be catching the train to work while her car's in the workshop
   does not imply a progressive action.  It implies a repeated future action because the verb in this sense cannot be seen as progressive; it is an instantaneous or momentary verb unlike, e.g., verbs such as work, read, study etc. which have a durative sense.
   A time line to show the concept might be
   
   It can also imply that this is a new arrangement rather than a one-off event and that fact is often signalled by an adverbial (a clause expressing reason in the example above) or a prepositional phrase as in, e.g.:
       She'll by working from the office upstairs from next Monday.
3. A current speculation
   This is the modal use of the auxiliary verb will which refers to the speaker's view of whether an event is likely to be true.  It is, to use the technical term, and example of epistemic modality and does not refer to the future at all.  An example is:
       I expect she'll be sitting on the train to work
   The use of an introductory verb or clause such as I expect to increase or tone down the certainty shown is commonplace so we may also have:
       I suspect she'll be be sitting on the train to work
   Other modal auxiliary verbs and expressions may be added to the mix to signal very subtle shades of perceived likelihood as in, e.g.:
       She may / might well be / must / could be sitting on the train to work
   and so on.
   A time line for this will show that the present is included in the speculation, like this:
   
   but is also shows that the speaker is uncertain how long the state will persist.
4. 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 soon 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, linked below, for more.

Whichever use we consider there is an overriding concept of looking back from a point in time after now and considering what has happened.  Any time line needs clearly to show the point of view of the speaker / writer.  As in, e.g.:

in which the future event is denoted by the figure 1 and the point in the future when it is being considered by the figure 2.

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.
A time line to show this sense might be:

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

This is clearly another example of where one action or state causes or is dependent on a previous one.  Again, however, the speaker wishes to emphasise the duration of the action.
A time line for this is different because the action is not seen to continue after the state of being tired but to have led up to and caused it.  It might look like this:

Use 3: We'll have been coming to this hotel for 15 years soon

This is what is known as an iterative form because the event is repeated.  In English, the iterative is often signalled by the use of the progressive form of the verb.  In other tenses, for example, we get:
    She's been telephoning repeatedly
    Someone is banging on the door
etc.
Iterative (or repeated) events are signalled by the -ing form of the verb in English with what are known as instantaneous verbs such as break, bang, glance, hit etc. because semantically these verbs cannot describe a progressive action.
Other verbs which are by nature durative, such as read, live, stay, wait etc. when they appear with the -ing form denote progressive or background events rather than repeated events.
A time line for this example might be:

and from that can be seen that this use of the future perfect iterative carries with it the use of the perfect aspect to embed the past and the future events in a more distant future event.  That is, for many learners, wholly non-intuitive.

There is a good deal more on the uses of these two tenses in the guide to speaking about the future.

The problems