Transform is a static ability that enables you to play certain creature cards as an instant or sorcery spell, rather than as a creature spell.
The official rules for transform are as follows:
502.99a Transform is a static ability that functions in any zone from which the card can be played. "Transform Instant" means "You may choose to play this card's instant spell mode rather than playing it as a creature spell." "Transform Sorcery" means "You may choose to play this card's sorcery spell mode rather than playing it as a creature spell." Playing a spell by choosing its transform ability mode follows the rules for paying alternative costs in rules 409.1b and 409.1f-h.
502.99b Since any spell with the transform ability is modal, the player announces the mode choice immediately after the player announces that he or she is playing the spell.
502.99c If the creature spell mode is chosen, the spell's card type, supertype, and subtype are those printed on the card. If the transform instant or sorcery spell mode is chosen, the spell's card type becomes instant or sorcery, and the supertype, subtype, power and toughness, and all rules text printed on the card above the transform ability are ignored. For example, if the transform sorcery spell mode of Antipodean is chosen when the card is played, the spell's card type becomes Sorcery, not Legendary Sorcery - Kraken.
502.99d If the spell targets one or more targets only if a particular mode is chosen for it, its controller chooses those targets only if he or she chose that mode. Otherwise, the spell is played as though it did not have those targets.
502.99e A card with the transform ability is put into play from the stack under the control of the spell's controller (if the creature spell mode was chosen) or is put into its owner's graveyard from the stack (if the transform instant or sorcery spell mode was chosen) as the final step of the spell's resolution.
502.99f Instants and sorceries can't come into play, so the "legend rule" for legendary permanents doesn't apply if a legendary creature is in play and another card with the same name is then played as a transform instant or sorcery spell.
* A card with the Transform ability works just like a normal modal card. As you play a card with the Transform ability, you choose which mode (creature or instant/sorcery) is being played. The choice is made when you play the spell. You can choose only one of a card's modes.
* You then choose any target or targets, but only if you chose the Transform instant or sorcery mode. If the Transform instant or sorcery mode of a card (such as Alluvial Kavu) has a target, you can't choose that mode unless you can choose a legal target for it.
* When you play a spell and choose its Transform mode, you really are playing the spell -- you're just choosing a different mode. The spell can be countered as normal. Effects that prevent you from playing a spell also prevent you from playing the Transform mode of the spell.
* Playing a card by choosing its Transform mode works just like playing any other card, with one exception: The creature's power and toughness, its card type, supertype and subtypes, and all rules text above the Transform ability are ignored.
* For each Prevenience creature with the Transform ability, choosing the creature spell mode just gets you the creature (and ignores the Transform language), while choosing the Transform mode just gets you the Transform instant or sorcery ability.
* When you play a spell and choose its Transform mode, its mana cost doesn't change. Effects that cause you to pay more or less for a spell (like the Leeches) will cause you to pay that much more or less while playing it in its Transform mode, too. That's because those effects affect the total cost of the spell.
* If you're playing a card with the Transform ability "without paying its mana cost," you can choose its normal creature spell mode or choose its Transform instant or sorcery spell mode, unless a particular card (such as Conversion Portal) prevents you from doing so.
* A card played as a Transform instant or sorcery spell follows the normal timing rules for instants and sorceries.
* The timing of a creature card with the Transform ability might change, depending on which mode you choose. If a creature card without flash has a Transform instant ability (such as Westwind Ancient), you can play the Transform instant ability on the card any time you could play an instant, but you can play the card as a creature only any time you could play a sorcery. If a creature card with flash has a Transform sorcery ability (such as Bog Defender), you can play the card as a creature any time you could play an instant, but you can play the Transform sorcery ability on the card only any time you could play a sorcery.
* If a player targets a Transform instant or sorcery spell with a Deflection-type spell or if a player takes control of the spell (with Mirror of Odin, for example), he or she can change the target of the Transform spell but not the mode of that card.
* There is a cycle of Legendary Dragons with the Transform ability (Ancestral Dragon, Armageddon Dragon, Immortal Dragon, Lorekeeper Dragon, and Twilight Dragon), and another cycle of Legendary creatures with the Transform ability (Machius, Aspect of Wrath, Antipodean, Harbinger of the Wastes, Earthbound Dragon, and Legacy of Karviddam). If an Armageddon Dragon or Antipodean is in play, another card with the same name can be played in its Transform sorcery mode without violating the "legend rule." When the Transform sorcery spell resolves, it is removed from the stack and put directly into its owner's graveyard, while the Legendary creature with the same name remains in play.