快速答案
causal 与因果有关；casual 与偶然、偶尔或非正式有关。
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
causal
causal
Use causal for relating to a cause-and-effect connection.
casual
casual or by chance
Use casual for happening occasionally, informally, or without prior planning.
如何选择
- causal: Choose causal if the sentence points to relating to a cause-and-effect connection.
- casual: Choose casual if the sentence points to happening occasionally, informally, or without prior planning.
例句
O estudo não provou uma relação causal.
The study did not prove a causal relationship.
Foi um encontro casual no aeroporto.
It was a chance encounter at the airport.
常见误区
A correlação causal explains why something happens. A coincidência casual simply happens without planning.
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 causal，一次用 casual。如果两个版本都可以，说明意思怎样变化。