怎样区分reduce,decrease,diminish,decline?

这一组动词都有"减少,减轻"的意思.
alleviate v.在痛苦方面的减轻,缓和
The painkiller alleviated the pain.止疼药减缓了疼痛.
diminish v.指因为不断消耗,在数量方面缓慢减少,也指在素质或者价值的下降.
The supply of oil has diminished because of the war.由于战争石油的供应减少了.
reduce v.指人为地使某物在数量或重量方面的减少或降低.
He reduced the amount of money they could spend.他缩减了他们的开销.
decrease v.指数量上的减少；力量或者强度的减弱.
The population decreased a lot last year.去年人口数量急剧下降.
decline v. (数目、价格、比率)下降；谢绝,婉言推辞；衰退,衰落.
Last year, the crime rate in Chicago has sharply declined.去年芝加哥的犯罪率明显下降.