快速答案
já 常表示变化已经完成；ainda 表示之前的状态仍在持续。
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
já
already, now, or right away
Use já for something completed earlier than expected or true by now.
ainda
still or yet
Use ainda for something continuing or not completed up to now.
如何选择
- já: Choose já if the sentence points to something completed earlier than expected or true by now.
- ainda: Choose ainda if the sentence points to something continuing or not completed up to now.
例句
Eu já terminei.
I have already finished.
Eu ainda estou estudando.
I am still studying.
常见误区
In questions, já asks whether it has happened already, while ainda asks whether it remains true.
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 já，一次用 ainda。如果两个版本都可以，说明意思怎样变化。