You use theoretically to say that although something is supposed to be true or to happen in the way stated, it may not in fact be true or happen in that way.
例句
Theoretically, the price is supposed to be marked on the shelf...照理说,价格应该标在货架上。Emigration was still theoretically a matter of choice.从理论上讲,移民仍是个个人选择的问题。