reduce是一种主观的减少、降低，主语是使减少的外力，to make something smaller or less in size, amount, or price。
decrease是一种客观的减少、下降，主语是减少的东西（size, amount, or price etc.）,to go down to a lower level, or to make something do this。
拓展资料
decrease
1、verb
~ (from sth) (to sth)
（使大小、数量等）减少，减小，降低
to become or make sth become smaller in size, number, etc.
[V] The number of new students decreased from 210 to 160 this year.
今年新生人数从210减少到160。
The price of wheat has decreased by 15%.
小麦价格降低了15%。
This species of bird is decreasing in numbers every year.
这种鸟的数量在逐年减少。
a decreasing population
逐渐减少的人口
[VN] People should decrease the amount of fat they eat.
人们应减少脂肪的摄入量。
反义词： increase
2、noun
[C, U] ~ (in sth) | ~ (of sth)
减少；降低；减少量
the process of reducing sth or the amount that sth is reduced by
同义词： reduction
There has been some decrease in military spending this year.
今年的军费开支有所减少。
a decrease of nearly 6% in the number of visitors to the museum
参观博物馆人数下降将近6%。、
reduce
1、verb
[VN] ~ sth (from sth) (to sth) | ~ sth (by sth)
减少，缩小（尺寸、数量、价格等）
to make sth less or smaller in size, quantity, price, etc.
Reduce speed now (= on a sign) .
减速行驶。
Costs have been reduced by 20% over the past year.
过去一年，各项费用已经减少了20%。
Giving up smoking reduces the risk of heart disease.
戒烟会减少得心脏病的风险。
The number of employees was reduced from 40 to 25.
雇员人数从40人减少到了25人。
The skirt was reduced to £10 in the sale.
在大减价期间，这条裙子减价到10英镑。
2、[VN , V] （使）蒸发
if you reduce a liquid or a liquid reduces , you boil it so that it becomes less in quantity。
to lose weight by limiting the amount and type of food that you eat.
a reducing plan
节食计划
4、[VN] (chemistry 化) 使还原
to add one or more electrons to a substance or to remove oxygen from a substance
compare oxidize.