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You can't put more than one +1/+1 counter on any one target using the support action.
Together Forever's activated ability checks whether the target creature has a counter on it as the ability is activated and as the ability resolves. If the creature loses its counters later in the turn, the delayed triggered ability will still return it to its owner's hand when it dies.
If a creature without counters receives enough -1/-1 counters to reduce its toughness to 0 or less, Together Forever's ability can't be activated before state-based actions put that creature into its owner's graveyard.
Support can target a creature another player controls.
If a spell with support has other abilities that target creatures, those abilities and the support ability can target the same creature.
If some, but not all, targets for a spell become illegal, the remaining targets are affected as appropriate. If all of a spell's targets become illegal, that spell doesn't resolve.
