hello, why do i never see future perfect tense? how to use it?

It's probably not as common as some of the other tenses, but it's not completely rare.
Here is an example: "I predict that by the time I arrive, Nan WILL HAVE SPENT all my money at the clothing store." 💸
Here's a good explanation from Education First: "The future perfect tense refers to a completed action in the future. When we use this tense we are projecting ourselves forward into the future and looking back at an action that will be completed some time later than now."
Here are some more examples:
- Cheer up: you WILL HAVE COMPLETED two-thirds of the program by this time next month.
- I know it's hard right now, but you WILL HAVE EARNED three degrees by the time most people earn just one.

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