Psychic Terrain has to be active when Future Sight hits, and will only boost it if the user is grounded at that time, even if it wasn't grounded when the move was used. If the user is currently switched out, it's always considered grounded unless it's a Flying type (a Pokemon's Ability and item aren't checked while it's inactive).
-
Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
Smogon Forums
Marty
Aurella
Okay, so to describe to what I think you are saying:
Levitate Bronzong uses Future Sight > Tapu Lele switches in on the turn that future sight will hit > since Levitate Bronzong (future sight user) isn't active on the field (since Tapu Lele is in currently) then terrain will boost the future sight attack since the original user of the move is not in field and therefore it's ungrounding ability isn't checked..
Levitate Bronzong uses Future Sight > Tapu Lele switches in on the turn that future sight will hit > since Levitate Bronzong (future sight user) isn't active on the field (since Tapu Lele is in currently) then terrain will boost the future sight attack since the original user of the move is not in field and therefore it's ungrounding ability isn't checked..
Aurella
.... checked during terrain/damage calculation.