At last firemen have put out a big forest fire inCalifornia. Since then, they have been trying tofind out how the fire began. Forest fire areoften caused by broken glass or by cigaretteends which people carelessly throwaway.Yesterday the firemen examined the groundcarefully, but were not able to find any broken glass.They were also quite sure that a cigarette end didnot start the fire. This morning, however, a firemanaccidentally discovered the cause. He noticed theremains of a snake which was wound round the electric wires of a 16,000-volt power line. Inthis way, he was able to solve the mystery. The explanation was simple but very unusual. Abird had snatched up the snake from the ground and then dropped it on to the wires. Thesnake then wound itself round the wires. When it did so, it sent sparks down to the groundand these immediately started a fire..
本文语法：现在完成进行时
一句话总结：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。
1、At last firemen have put out a big forest fire in California.
消防队员们终于扑灭了加利福尼亚的一场森林大火。
语言点 put out = extinguish 扑灭
2、Since then, they have been trying to find out how the fire began.
从那时起，他们一直试图找出起火的原因。
语言点 辨析：begin, start
begin强调从零开始，反义词是end。
start常常是指中断后又开始，反义词是stop。
3、Forest fires are often caused by broken glass or by cigarette ends which peoplecarelessly throw away.
森林火灾时常由破碎的玻璃渣子或人们随手扔掉的烟头引起。
语言点1 本句中which引导定语从句。
语言点2 broken过去分词作定语，修饰glass。因为碎玻璃可以引起光的折射从而导致火灾的发生。
