英语时态疑问,肯体如下

英语时态疑问,肯体如下At last firemen have put out a big forest fire in California. Since then they have been trying to find out how the fire began. Forest fires are often caused by broken glass or by cigarette ends which people carelessly throw away. Yesterday, the firemen examined the ground carefully, but were not able to find any broken glass. They were also quite sure that a cigarette end did not start the fire. This morning, however, a fireman accidentally discovered the cause. He noticed the remains of a snake which was wound round the electric wires of a 16000-volt power line. In this way, he was able to slove the mystrey. The explaination was simple but very unusual. A bird had snatched up the snake from the ground and then dropped it on to the wires. The snake then wound itself round the wires. As it did so, it sent sparks down to the ground and these immediately started a fire.上面这篇文章取自新概念二册第53课,请问文章中为什么前面部分为什么要用现在完成时,明显根据文章得知,火在后面提到的检查地面,找到电线上的蛇这些事之前就扑灭了,按规则来讲应该用过去完成时和现在完成进行时啊,为什么?如果说是因为要强调对现在的影响,那就能视时态不顾了吗,那不就可以乱用了吗. 另外,单看文章前两句中,since then 从那时起,都说了从那时起,前面那句肯定是过去,为什么前面还用现在完成时,. The seed was sprayed over the ground in huge quantities by aeroplanes. The planes had been planting seed for nearly a month when it began to rain. By then, however, in many places the grass had already taken root. In place of the great trees which had been growing there for centuries patches of green had begun to appear in the blackened soil. 这篇文章中后面的had been growing there 可以用had been there 或had grown there 取代吗,为什么,或者说含义有什么不同,请说详细一点,最好可以让人有一种矛塞顿开的感觉,