used in describing what is supposed to happen or be possible, usually with the implication that it does not in fact happen
理论上（应该）；照道理
in theory, things can only get better; in practice, they may well become a lot worse.理论上情况只会变好，实际上却很可能变得更糟。
in theory, things can only get better; in practice, they may well become a lot worse.
理论上情况只会变好，实际上却很可能变得更糟。