While it is not one of the most commonly used verb forms, the future perfect continuous does occur quite often in conversations between advanced English speakers, and in classes and exams it shows knowledge of the language that is above average for most students.
The verb form is used to describe events that will start at one point in time and continue until a second point in time in the future. This second point is usually mentioned explicitly.
This timeline shows what we mean:
For example, imagine two people are having a conversation about an upcoming road trip. They are discussing when they should take a rest stop.
A: How about we stop around Sheffield to eat?
B: That’s a good idea. By then, we will have been driving for three hours.
Notice that Person B is describing the continuous length of time it will take them to drive to Sheffield from their starting point. This is the essence of the future perfect continuous. It describes a continuous activity that will take place between one action (starting the journey) and another (stopping to eat).
Another example could be used during a friendly football game.
A: Why don’t we take a break?
B: Let’s wait until it gets dark.
A: But we will have been playing for hours by then!
Notice again that the form is used to measure a length of time between two actions, in this case when they started to play football and when they intend to take a break. This is probably the most common usage of the future perfect continuous, but it can be used for other non-time related situations also, though much less commonly.
A: Let’s go check on the party decorators.
B: Sure. They will have been preparing the room with the balloons and lights.
The future perfect continuous is made up of the following structure:
will have been + present participle
First comes the subject of the sentence and then the verb form and there is often a complement following the present participle though this is not always necessary. These examples don’t have a complement (although it’s often implied or suggested by the context):
They will have been eating.
She will have been swimming.
My band won’t have been playing.
These examples include a complement:
They will have been eating for hours by the time we get to the restaurant.
She will have been swimming for a while before she gets tired.
My band won’t have been playing without a drummer.
This last example brings us on to the negative form of the future perfect continuous. In this case not is added after the will.
will + not + …
I will not have been looking at it by then.
We will not have been searching for long.
They will not have been drinking too much.
When the negative future perfect continuous tense is used in speech (but not formal writing), will + not is nearly always contracted.
We won’t have been buying much by the afternoon.
I won’t have been writing a lot.