快速答案
algum 表示存在某个未确定的对象；nenhum 表示一个也没有。
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
algum
some or any
Use algum for an unspecified person or thing, often in questions.
nenhum
no or not any
Use nenhum for the absence of even one person or thing.
如何选择
- algum: Choose algum if the sentence points to an unspecified person or thing, often in questions.
- nenhum: Choose nenhum if the sentence points to the absence of even one person or thing.
例句
Você tem algum irmão?
Do you have any brothers?
Não tenho nenhum irmão.
I do not have any brothers.
常见误区
After a negative verb, nenhum is natural in Brazilian Portuguese. The double negative reinforces one negative meaning.
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 algum，一次用 nenhum。如果两个版本都可以，说明意思怎样变化。