第一句：The villagers didn’t realize how serious the pollution was until all the fish died in the river.
第二句：Not until all the fish died in the river did the villagers realize how serious the pollution was.（倒装句）
第三句：Not until his son came back from school did he go to bed.
第四句：It was not until his son came back from school that he went to bed.（强调句）
强调句：在英文中，句子的主语、宾语、状语都可以写进一定的句式里以表示对它的强调。
倒装句：英语的基本语序是“主语谓语”，假如将谓语的一部分或全部放在主语之前，这种语序叫倒装。倒装既是一种语法手段，也是一种修辞手段，用于表示一定的句子结构或强调某一句子成分。
It was not until all the fishes in the lake died that villagers began to realize how severe the pollution was.
Not until all the fishes in the lake died did villagers begin to realize how severe the pollution was.
It was not until his son left school that he went to sleep.
Not until his son left school did he go to sleep.
Villagers have not realized that the pollution was so serious until all fishs on the lake died.
Not until all fishs on the lake died did villagers realize that the pollution was so serious.
Not until his father had come back did he go to sleep
He hadn't gone to bed until his son came back.
Not until his son had come back did he go to sleep