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. Please excuse my opening your letter by mistake. 请原谅我误拆了你的信。
2. Excuse me my friend. 原谅我我的朋友。
3. No excuse to absent. 没有缺席的理由。
英英解释
n.
1. a defense of some offensive behavior or some failure to keep a promise etc.
2. a note explaining an absence
3. a poor example v.
1. accept an excuse for
2. grant exemption or release to
3. serve as a reason or cause or justification of
4. defend, explain, clear away, or make excuses for by reasoning
5. ask for permission to be released from an engagement
6. excuse, overlook, or make allowances for; be lenient with
