Use the future perfect tense to express an idea that something will end before another action in the future. Therefore, there are two future actions.
An action before another action (in the future)
- He will have finished writing the letter (Action 1) by the time his father gets home. (Action 2)
- We will have completed our work (Action 1) when you come. (Action 2)
- I will have gone home (Action 1) when you come to my office at 5:00 PM. (Action 2)
As you can see, there are two actions – Action (1) and Action (2). The sentence in Example #1, for instance, indicates that when his father gets home, the letter is already complete.
You have to use “will have” followed by the past participle verb form for the first action, while the second action uses the simple present verb form.