快速答案
diminuir 常表示某物自行变少；reduzir 常表示主动把某物降到较低水平。
可靠的方法是先判断这个词在句子里的功能，再翻译。
每个词的作用
diminuir
to decrease, lessen, or become smaller
Use diminuir for a quantity, sound, speed, pain, or intensity going down.
reduzir
to reduce or cut
Use reduzir for deliberately bringing a quantity, cost, risk, or scope down to a lower level.
如何选择
- diminuir: Choose diminuir if the sentence points to a quantity, sound, speed, pain, or intensity going down.
- reduzir: Choose reduzir if the sentence points to deliberately bringing a quantity, cost, risk, or scope down to a lower level.
例句
A chuva diminuiu à tarde.
The rain eased in the afternoon.
Precisamos reduzir o desperdício.
We need to reduce waste.
常见误区
Both can be transitive or intransitive in many contexts. Reduzir sounds more planned or technical; diminuir is broader in daily speech.
放到语境中练习
从你的真实生活中选一个句子，写两遍：一次用 diminuir，一次用 reduzir。如果两个版本都可以，说明意思怎样变化。