cause 指直接导致事情发生的原因，与所发生的事个有因果关系。
reason 强调指从逻辑推理上得出的结论性原因，不是直接说明起因。
excuse 指为自己辩解，希望得到谅解时所提出的种种理由，是借口性的理由。
参考例句
1. The resignation of the prime minister will cause much confusion. 首相的辞职将引起很大混乱。
2. What was the cause of the accident? 造成这一事故的原因是什麽?
3. What cause is I self knows neither. 原因是什么我自己也不知道。
英英解释
n.
1. events that provide the generative force that is the origin of something
2. a justification for something existing or happening
3. a series of actions advancing a principle or tending toward a particular end
4. any entity that produces an effect or is responsible for events or results
5. a comprehensive term for any proceeding in a court of law whereby an individual seeks a legal remedy v.
1. give rise to; cause to happen or occur, not always intentionally
2. cause to do; cause to act in a specified manner
