theories
A theory is a formal idea or set of ideas that is intended to explain something.
Marx produced a new theory about historical change based upon conflict between competing groups.
基于对立集团之间的冲突,马克思提出了一个关于历史变革的新理论。
Einstein formulated the Theory of Relativity in 1905.
爱因斯坦于1905年系统地提出了相对论。
If you have a theory about something, you have your own opinion about it which you cannot prove but which you think is true.
There was a theory that he wanted to marry her.
那时有传闻说他想娶她。
The theory of a practical subject or skill is the set of rules and principles that form the basis of it.
He taught us music theory.
他教我们乐理。
in theory
You use in theory 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.
Achieving these goals is relatively easy in theory, yet quite difficult in practice.
实现这些目标从理论上说比较容易,但实际上却相当困难。
A school dental service exists in theory, but in practice, there are few dentists to work in it.
学校理论上有个牙医诊所,但事实上里面的牙医寥寥无几。
In theory, the technology is straightforward.
理论上讲,该项技术很简单。