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...照理說，價(jià)格應(yīng)該標(biāo)在貨架上。Emigration was still theoretically a matter of choice.從理論上講，移民仍是個(gè)個(gè)人選擇的問題。