This is not the case. With cards like Izzet Charm, you must choose your "Mode" as a part of the initial casting of the spell. This is locked in to the spell once it's on the stack, and you can't change it later. This is important for things like copying spells (a copied spell has the same modes), and as you show in your example, knowing how to respond to one. Your opponent gets to know what mode you chose for charm before he or she responds.
From the comp rules:
(on how to cast a spell):
601.2b If the spell is modal the player announces the mode choice (see rule 700.2).
(on modal things in general):
700.2a The controller of a modal spell or activated ability chooses the mode(s) as part of casting that spell or activating that ability. If one of the modes would be illegal (due to an inability to choose legal targets, for example), that mode can't be chosen. (See rule 601.2b.)
This is not the case. With cards like Izzet Charm, you must choose your "Mode" as a part of the initial casting of the spell. This is locked in to the spell once it's on the stack, and you can't change it later. This is important for things like copying spells (a copied spell has the same modes), and as you show in your example, knowing how to respond to one. Your opponent gets to know what mode you chose for charm before he or she responds.
All right. Thank you. Will you provide the section in the comprehensive rules for future reference?
Am I correct when I say the following statements?
1: I do not have to choose what my Izzet Charm does until it is declared to have resolved
2: Once it has been declared to have resolved, my opponent is unable to Unsummon his Delver of secrets.
If possible, would you tell me the places in the comprehensive rules that it states these things?
From the comp rules:
(on how to cast a spell):
601.2b If the spell is modal the player announces the mode choice (see rule 700.2).
(on modal things in general):
700.2a The controller of a modal spell or activated ability chooses the mode(s) as part of casting that spell or activating that ability. If one of the modes would be illegal (due to an inability to choose legal targets, for example), that mode can't be chosen. (See rule 601.2b.)
All right. Thank you. Will you provide the section in the comprehensive rules for future reference?