I don't know what you mean by "sorcery speed," though I do now agree that treating formshift like an Instant is too powerful, even if the transformations in-comic seem to be (close to) instantaneous.
(And, lack of MTG know-how again: dunno what you mean by "stack")
So you have, essentially, 2 'speeds' of cards/abilities you can play. Instants, and sorceries (of course, there are other limiters and whatnot, but this just in a general sense.) Instants are usually
cheaper, temporary effects and can be played any time you have priority. Priority is passed at every new phase (upkeep, draw, main 1, combat, main 2, end) and after every played card or other action that a player takes. The active player (the one who's turn it is) always has first priority, and then passes it to the next player. An instant-speed card/ability may be played any time you have priority. A sorcery-speed card/ability may only be played during your first or second main phase, and only on your turn. It may NOT be played in response to another spell.
Now, the stack is what happens when you play a spell, and someone else plays an instant-speed card/ability in response. For example: say I have a 2/2 artifact creature in play. It's my turn, and I pass priority from my 1st main phase so that I can enter combat. Now that he has priority, my opponent casts "Lightning" for 3 damage targeting my 2/2, which would normally kill my creature. However, I respond to that by casting Giant Growth (+3/+3) now there are 2 spells on the stack, neither of which have resolved. My opponent could respond again with a Counterspell, another damage spell, or anything really and add another spell to the stack. When no one has any more spells to cast you 'resolve' the stack, starting at the last spell played. In my case the Giant Growth would be applied first, making my 2/2 a 5/5, then the 3 damage: which isn't enough to kill it.
Note: If I had cast Giant Growth first, and he responded with Lightning Lightning would resolve before GG, and I would have a dead creature, which gives GG no target, and simply sends it to the graveyard.