say my opponent declares the end of a main phase and I "instant" something at the end of said phase. Does my opponent still have the option to cast sorcery-speed stuff at end of phase?
Yes. Steps and phases don't end until both players pass priority with the stack empty. If your opponent passes priority intending to end the phase and you cast an instant, the active player will once again have priority after that instant resolves. The stack will be empty, so s/he can do things that can only be done with the stack empty during a main phase (casting an artifact/creature/enchantment/sorcery spell, for instance).
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Lapsed DCI Rules Advisor - Retired from playing but still hanging around
Yes. When you cast an instant, it is still in the main phase, and you are interrupting the shortcut to move to another phase. Because of the interruption, the phase stays in main, and the active player will have a chance to cast things that arent instant/flash
W may only be paid with white mana. U may only be paid with blue mana. B may only be paid with black mana. R may only be paid with red mana. G may only be paid with green mana. C may only be paid with colorless mana. 1 may be paid with white, blue, black, red, green, or clolorless mana.