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"they will remain long after both of us have left the planet."
This is a future-past sentence?
"they will remain long after both of us will leave/leave the planet "
which one would be the correct one?
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Picnic"they will remain long after both of us have left the planet."This is a future-past sentence?
This sentence is okay. Yes, it refers to a future when some future event (our leaving) is already past.

"they will remain long after both of us leave the planet "
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PicnicThis is a future-past sentence?
I'm not familiar with the term "future-past sentence", but if it helps you understand something, then it's OK with me. Two of four choices are OK. See below.

They will remain long after both of us

leave present

will leave future

have left present perfect

will have left future perfect

the planet.

CJ
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Comments  
What tense is used here?
It's like:" by the time you leave we will have land already " right?
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PicnicWhat tense is used here? It's like:" by the time you leave we will have land already " right?
Future perfect.

The future perfect group "will have already" is pretty standard with "by the time".

By the time you leave we will have already landed in Jamaica.
By the time I get there the tickets will have already been sold out.
By the time they solve this math problem, I'll have already solved five others.

(You can also position already so it's before have.)

CJ