The future perfect tense is formed by using the simple future tense of the verb have (will have) + the past participle of the main verb. It is used for an action which will have finished by some future time or date as mentioned.

Examples:

  • Statement: She will have sewn the patch on my jeans by tonight.
  • QuestionWill the people have put out the fire by the time the firemen arrive?

 

The future perfect tense is used:

to show that an activity will be completed by a specified time in the future.

Examples:

  • You can come over tomorrow evening as you will have completed your work by then.
  • will have saved about one million dollars by the year 2090.

 

to show that an action will be completed before another takes place in the future.

Examples:

  • The fire will have burnt the building to the ground by the time the firemen arrive.
  • He will have completed his homework before he goes to bed.

 

to show a situation that will be over in the future.

Examples:

  • The special offer – buy two, get one free – will have ended by the weekend.
  • They will have demolished that old building across the road by midday tomorrow.

 

with conditional if.

Examples:

  • If you don't hurry up, they will have eaten all the food when you get there.
  • I will have finished painting the fence within two hours if it doesn’t rain.

 

with time phrase/clause. 
The future perfect tense may come either before or after the time clause. A comma is usually placed at the end of a time clause when the time clause comes before the main clause.
 

Examples:

  • On April 1st, she will have stayed with me for six months.
  • She will have stayed with me for six months on April 1st.
    (The time phrase is on April 1st, and the main clause is she will have stayed with me for six months.)
  • By the time he joins us, we will have been in the library for an hour.
  • We will have been in the library for an hour by the time he joins us.
    (The time clause is by the time he joins us, and the main clause is we will have been in the library for an hour.)

 

with time expressions such as by seven o'clock, by this evening, by next Thursday, by then, until noon tomorrow, before closing date.

Examples:

  • We will have prepared the documents by next Friday.
  • I will have woken up before the sun rises tomorrow.

 

Future perfect tense and future perfect continuous tense
Both these tenses convey the same meaning as show by this example.
 

Examples:

  • When Joan competes in the marathon next week, she will have trained for nine months.
  • When Joan competes in the marathon next week, she will have been training for nine months.