"they will have forgotten us already" - Why not?
Should be will have already...will and have should not be separated
Duo accepted "will have already" but I never learned that will and have shouldn't be separated, so I see nothing wrong with "will already have".
What's wrong with "when we'll be old they'll have forgotten us already"?
כְּשֶׁנִּהְיֶה זְקֵנִים הֵם כְּבָר יִשְׁכְּחוּ אוֹתָנוּ