用強(qiáng)調(diào)句表示原有的意思,時(shí)態(tài)是不能隨意改變的,也就是說,原來不強(qiáng)調(diào)的句子是現(xiàn)在時(shí)態(tài)就用 is,原來的句子是過去時(shí)態(tài)則用 was.
由上可以推斷,你所提到的兩個(gè)句子都是過去時(shí),因此只能依舊保持原來的時(shí)態(tài)不變：
1.Mrs.Williams didn't know anything about it until I told her.
可以有以下兩種改法：
—— It was until I told her that Mrs.Williams didn't know anything about it.(用 it is ...that 句型強(qiáng)調(diào))
—— Not until did I told her Mrs.Williams knew anything about it.（用倒裝句型強(qiáng)調(diào)）
2.We could not cross the river because the water had risen.
—— It was because the water had risen that we could not cross the river