what is future perfect simple ??

Simple future and future perfect are two separate forms of future tense.

Simple future: It talks about future to state a fact that will be accomplished in future.

For example: I shall visit the doctor next week.

Future perfect: It expresses events that will be completed by a certain time in future.

For example: I shall have visited the doctor by next month.

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The future perfect simple - grammar rules

Form

Positive statement: I will have worked, I will have written, He will have worked, He will have written (I'll have worked, He'll have worked)
Negative statement: I will not have worked (I won't have worked), He will not have worked (He won't have worked)
Question form: Will you have worked?
Negative question: Will you not have worked? (Won't you have worked?)

We make it with will + have + past participle. The past participles are different for regular and irregular verbs.

Passive voice: The label will have been removed. The lawns will have been mown. (See more at active and passive voice.)

Use

We use the future perfect simple tense for activities 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 ... On the other hand, you must be careful with other time expressions, because this tense cannot be used in time clauses with expressions such as when, while, before, after, as soon as, if, unless, etc., which are normally used in the time clauses with other tenses.

The time can also be given by other time expressions (on Sunday, before 31 June) or actions.

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 actions will be in the past.

The future perfect continuous - grammar rules

Form

Positive statement: I will have been writing (I'll have been writing)
Negative statement: I will not have been writing (I won't have been writing)
Question: Will you have been writing?
Neg. question: Will you not have been writing? (Won't you have been writing?)

Use

We use the future perfect continuous tense for activities that will continue until a point of time and will not be completed. It is also normally used with by or other time expressions and future events.

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.

Comparison

The continuous is used for incomplete, uninterrupted actions. If we refer to a number of individual events or events that were repeated, we must use the 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)