These are all words for a cause or an explanation for sth that has happened or that sb has done. 以上各词均指事情发生或做某事的原因、理由、解释。

• reason a cause or an explanation for sth that has happened or that sb has done; a fact that makes it right or fair to do sth 指某事发生或做某事的原因、理由、解释、道理：◆He said no but he didn't give a reason. 他说不行，但没有说明原因。
• explanation a statement, fact or situation that tells you why sth has happened; a reason given for sth 指解释、说明、阐述：◆The most likely explanation is that his plane was delayed. 最可能的解释是他的飞机晚点了。◆She left the room abruptly without explanation. 她未作解释就突然离开了房间。
• grounds (rather formal) a good or true reason for saying, doing or believing sth 指说、做或相信某事的充分理由、根据：◆You have no grounds for complaint. 你没有理由抱怨。
• basis (rather formal) the reason why people take a particular action 指原因、缘由：◆On what basis will this decision be made? 将基于何种原因作出这一决定呢？
• excuse a reason, either true or invented, that you give to explain or defend your behaviour; a good reason that you give for doing sth that you want to do for other reasons 指为自己行为所作的辩护、借口、理由：◆Late again! What's your excuse this time? 又迟到了！你这次有什么借口？◆It gave me an excuse to take the car. 这使我有理由开车去了。
• motive a reason that explains sb's behaviour 指动机、原因：◆There seemed to be no motive for the murder. 这起谋杀案看不出有什么动机。
• justification (rather formal) a good reason why sth exists or is done 指事物存在或做某事的正当理由：◆I can see no possible justification for any further tax increases. 我看不出还能提出什么理由来加税了。

grounds or justification? 用 grounds 还是 justification？

Justification is used to talk about finding or understanding reasons for actions, or trying to explain why it is a good idea to do sth. It is often used with words like little, no, some, every, without, and not any. Grounds is used more for talking about reasons that already exist, or that have already been decided, for example by law: moral/economic grounds. * justification 用来表示找出或明白做事的理由，或解释要做的事，常与 little、no、some、every、without 和 not any 等词连用。grounds 多指已存在的原因或已决定的（如法律上的）根据：moral/economic grounds（道德/经济原因）。

• pretext (rather formal) a false reason that you give for doing sth, usually sth bad, in order to hide the real reason 指为掩盖做某事（通常为不好的事）的真正理由而找的借口、托辞：◆He left the party early on the pretext of having to work. 他借口有事要处理，早早离开了聚会。

Patterns

• (a/an) reason/explanation/grounds/basis/excuse/motive/justification/pretext for sth
• the reason/motive behind sth
• on the grounds/basis/pretext of/that…
• (a) good/valid reason/explanation/grounds/excuse/motive/justification