1. 理论;学说
A theory is a formal idea or set of ideas that is intended to explain something.
e.g. Marx produced a new theory about historical change based upon conflict between competing groups...
马克思创立了一种新理论，认为历史的演变是基于对立集团之间的冲突之上的。
e.g. Einstein formulated the Theory of Relativity in 1905.
爱因斯坦于 1905 年创立了相对论。
2. 观点;见解
If you have a theory about something, you have your own opinion about it which you cannot prove but which you think is true.
theory的解释
e.g. There was a theory that he wanted to marry her...
有人认为他想娶她。
e.g. My theory about divorce is that it's not the split-up that damages children, it's how it is handled.
我对离婚的看法是，伤害孩子的不是婚姻的破裂，而是处理离婚的方式。
3. (实践科目、技能的)理论基础
The theory of a practical subject or skill is the set of rules and principles that form the basis of it.
theory的反义词
e.g. He taught us music theory.
他教我们乐理。
e.g. ...graduates who are well-trained in both the theory and practice of statistics.
在统计学理论和实践两方面都训练有素的毕业生
4. 从理论上说;照理说
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.
e.g. A school dental service exists in theory, but in practice, there are few dentists to work in them.
照理说，学校也有牙医诊所，可实际上，里面的牙医寥寥无几。