1. 语法讲解：
现在完成进行时
现在完成进行时表示过去某时开始发生，一直延续到现在并且可能继续 发生下去的动作。现在完成进行时即现在完成时和进行时的复合时态（has/havebeen + doing）。
How long have you been walking?
你走了多长时间？
He has been thinking of how to succeed.
他一直在想如何获得成功。
2. They were also quite sure that a cigarette end did not start the fire.
他们还十分肯定火灾也不是由烟头引起的。
知识点：sure的三种用法：1) sb. be sure of某人确信；2) sb. be sure to do某人相信能够去做某事；3 ) sb. be sure that某人确信
3. The explanation was simple but very unusual.
解释简单，却异乎寻常。
知识点：这是一个省略句，补充完整为：The explanation was simple but was very unusual.
4. When it did so, it sent sparks down lo the ground and these immediately started a fire.
当它这样做时，把大花送到了地面，这法火花立刻引起了一场大火
知识点：so常常用在动词后表示前句说过的一件事情或者某个动作，避免重复。
