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。

There was a theory that he wanted to marry her.

那时有传闻说他想娶她。

He taught us music theory.

他教我们乐理。

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。