这句话的主语是 “她”,构不成从句,应该翻译为 She didn't know there was little water left in the glass. 如果用主语从句翻译为 That there was little water left in the glass was not known by her 反而显得别别扭扭。
想用就用,这是很随意的事情。就像你说话一样,难道你在说话前还要思考用什么句式吗?顾名思义,主语从句是一个句子做了另一个句子的主语。当你发现你想表达的句子中的主语是一个句子时,很自然而然的就会想到用主语从句。怎么翻译呢?一般“that”开头做引导词的那种,直接顺着译就行(不用译that,因为它无实际意义)。另一种以“what,who等疑问词”开头,只要把它放回它原本在的位置,再顺着译就可以。如“what I want to do is to have breakfast。”“what”原本在do后面做do的宾语,缺了what,句子会成分不全。放在句首为了说明是主语从句,放在原位置就会使句子结构混乱。
What he doesn't know is that there isn't much water in the cup.
She didn't know that there was not much water in the glass.