Do you remember what the past perfect tense is used for? This tense is used to describe an event that happened before another one in the past. If you say I had already got home before it began to rain, for instance, that means you got home first, then perhaps a few minutes later it started to rain. If you understand this concept, you will easily understand the future perfect tense.
The past perfect tense and the future perfect tense are similar in that both describe an event that happens before another one a specific time. The difference is that the past perfect tense is for past events, and the future perfect tense is for future evennts.
The followings are two examples:
1. Andi will have already left before Tina arrives.
Imagine the event happens tomorrow. The sentence means when Tina arrives, Andi is not there because he leaves before Tina’s arrival.
2. The concert will have started when you get there.
Imagine the event happens tonight. The sentence implies that the concert will start earlier than your arrival at the concert.
Remember that you use will + have + past participle (3rd form of the verb).