Future perfect tense is used to describe an action or event completed before a future time or an action before a future action or event.
Future perfect simple
Positive statement: I will have painted, I will have written, He will have painted, He will have written (I’ll have painted, He’ll have painted)
Negative statement: I will not have painted (I won’t have painted), He will not have painted (He won’t have painted)
Question: Will you have painted?
Neg. question: Will you not have painted? (Won’t you have painted?)
We use the future perfect simple for events that will be completed before or at a certain time. It is often used with a time expression beginning with by: by then, by that time, by midnight, by the end of the year. The time can also be given by other time expressions (on Sunday, before 31 June) or other activities expressed in different future tenses.
I will have sent the project by Friday.
On 11 August this year we will have been married for five years.
When the mountaineers get back to the base, they’ll have been in the snowstorm for two days.
We’ll have reached the top before noon.
How long will she have worked here by the end of this year?
In all these examples, at a given time the future perfect actions will be in the past.
Future perfect continuous
Positive statement: I will have been meeting (I’ll have been meeting)
Negative statement: I will not have been meeting (I won’t have been meeting)
Question: Will you have been meeting?
Neg. question: Will you not have been meeting? (Won’t you have been meeting?)
We use the future perfect continuous tense for activities that will continue until a point of time in the future and will not be completed. Like the simple tense it is normally used with by or other time expressions and future actions.
I’ll go home on 20 June. By then I’ll have been staying at this hotel for a fortnight.
At six o’clock we’ll have been waiting here for three hours.
When you arrive, we’ll have been sitting in the classroom all day.
Future perfect simple vs continuous
It is used for incomplete, uninterrupted activities. If we refer to a number of individual actions or actions that were repeated, we must use the future perfect simple.
When I am sixty, I’ll have been building houses for thirty years. (one incomplete activity)
When I am sixty, I’ll have built more than fifty houses. (fifty individual actions)
By 5 o’clock I’ll have been washing this car for an hour and a half. (one uninterrupted activity)
By 5 o’clock I’ll have washed this car and replaced the tyres. (two completed activities that will be done one after another)
In this respect the simple and continuous aspects are similar to the other tenses (the past tense, present perfect, past perfect), which you can study on this website to get more details and more examples.