Moss-Pit Skeleton's last ability triggers if a creature enters the battlefield under your control with +1/+1 counters on it, as well as when +1/+1 counters are put on a creature you already control.

Moss-Pit Skeleton's last ability triggers only if it's already in your graveyard as +1/+1 counters are put onto a creature you control.

If you put a permanent with a kicker ability onto the battlefield without casting it, you can't kick it.

If a spell's kicker cost was paid, the spell is "kicked."

If a card or token enters as a copy of a permanent, the new permanent isn't kicked, even if the original was.

If you copy a kicked spell on the stack, the copy is also kicked. If the copied spell is a permanent spell, the token the copy of that spell becomes when it enters is also kicked.

To determine a spell's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases (such as kicker), then apply any cost reductions. The spell's mana value remains unchanged, no matter what the total cost to cast it was.

The kicker ability doesn't let you pay a kicker cost more than once.