not until.heard from
until 用于强调时间的强调句型：
It is (was) not until … that " 直到 … 才 " ,此时不可以用 till 代替.
It was not until yesterday that I got your letter/heard from you.
直到昨天,我才收到你的来信.
强调句型
It is (was) +被强调部分+ that (who) + 句子其他部分.(当强调的部分是人,且为句子的主语时,才用 "who",其余用that.)
强调句去掉It is (was)...that 之后还是一个完整的句子.
此结构强调的成分仅限于主语,宾语和状语.
I原句：My father did the experiment in the lab yesterday evening.
强调主语：It was my father who did the experiment in the lab yesterday evening.
强调宾语：It was the experiment that my father did in the lab yesterday evening.
强调时间：It was yesterday evening that my father did the experiment in the lab.(注意不用when)
强调地点：It was in the lab that my father did the experiment yesterday evening.