语法总结
Since then + 现在完成进行时
Since then, they have been trying... 表示“从那以后一直在努力”，既写出起点，也写出持续过程。
相似句：
- Since then, we have been searching for the answer.
- Since that accident, he has been driving much more carefully.
be not able to / be able to
were not able to find... 和 was able to solve... 在同一课里形成前后对照，突出“先找不到，后来终于找到”。
相似句：
- At first we were not able to open the box.
- In the end, he was able to explain the mystery.
过去完成因果链
A bird had snatched up the snake... then dropped it... 这种连续动作很适合用先后顺序复述因果链，帮助把“为什么会起火”说清楚。
相似句：
- The bird had caught the snake before it dropped it on the wires.
- Because the snake had wound itself round the wire, sparks fell to the ground.