1.lessen v. become less；reduce sth
eg: 1)The pain was already lessening.
1.I hope to obtain your forgiveness, to your opinion.
（强调”reduce"）译：我希望得到你的原谅，减轻你对我的看法。
2.decreasev. to become less in number or amount:
eg:1)The average rainfall has decreased by around 30 percent.
2) The effectiveness of the drug gradually decreases.
The number of nations allied with Germany as the war continued. （表示“数量”的减少）
译：战争继续进行，德国的同盟国减少。
3.dropv. to decrease, especially by a large amount: (= fall)
eg: 1)The number of tigers in the wild has fallen/dropped to just over 10,000.
2) At night, the temperature drops to minus 20 degrees.
3) Profits fell from ?98.5 million to ?76 million.
4) In May, the price of coffee dropped by over 20%.
The incidence of malaria dramatically because of eradication of malarial mosquitos.（表示“数量”的大幅减少，因为句中有一个词“dramatically”） 译：因为消灭了毒蚊子，所以痢疾的发生率大幅下降。
4.dwindle v. to gradually decrease until there is very little left of something. Dwindle is used about numbers or amounts. It is also used when saying that somethingbecomes less popular, less importantetc:
eg: 1)The birds' numbers have slowly dwindled, until there are now only about 600 left in the world.
2) Traffic along the canals dwindled during the 20th century.
Classes as more and more students played truant.（表示“数量”的减少，且这个过程是“渐渐的”直至只有很少一部分留下） 译：越来越多的学生旷课，所以班级数量逐渐减少。