英语He failed the examination because he was careless

很高兴能为您解答
首先我们来看看二者的区别
1、 for（因为）的用法：
for是并列连词,表示推断的原因,或对前一分句补充说明理由.
例：It must be morning for the birds are singing.一定是清晨了,因为鸟儿在叫.
2、 because （因为）的用法：
because通常表示直接的原因.because引导的从句表示直接而明确的原因和理由,表示按因果关系的推断,语气很强,用来回答why的问题,一般位于主句之后.
例：We couldn't go out because it was too cold.因为天气太冷,我们不能外出.
而且呢for 是介词,后面只能接名词性成分；because 是 连词后面只能接句子.
若非要换的换就换成：
He failed the examination for his carelessness.
He failed the examination for being careless.
希望能帮到您

because是从属连词，中间无标点符号。
for是并列连地，前有逗号。
because是原因最直接最强的语气，但是for是表示推测的原因
eg：It rained last night, for the ground is wet.

he failed the exam for his carelessness