After the fire

What was the danger to the villages after the fire?

  Firemen had been fighting the forest for nearly three weeks before they 

could get it under control. A short time before, great trees had covered the

countryside for miles around. Now, smoke still rose up from the warm ground 

over the desolate hills. Winter was coming on and the hills threatened the 

surrounding villages with destruction, for heavy rain would not only wash away

 the soil but would cause serious floods as well. When the fire had at last 

been put out, the forest authorities ordered several tons of a special type of 

grass-seed which would grow quickly. 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. 

原句：

1、When the fire had at last been put out, the forest authorities ordered several tons of a special type of grass-seed which would grow quickly.

在大火最后被扑灭后，森林管理当局订购了好几吨一种生长迅速的特殊类型的草籽。

2、The planes had been planting seed for nearly a month when it began to rain.

飞机撒播近一个月后，开始下起雨来。

第一个疑问：第一个when是after的意思，第二个when是有before的意思，为什么还有用when？岂不是误人子弟！虽然主从句时态告诉了我们。

原句：

3、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. 

然而到那时，很多地方的草已经生了根。一片片的绿草开始出现在这片烧焦的土地上，代替了多少世纪以来一直生长在那里的参天大树。

我的第二个疑问：

（1）都“by then”即都那时了 播种完了，怎么还是过去完成时？按理来说过去时才对，应该改成:

By then, however, in many places the grass already took root. In place of

the great trees which grew there in several  centuries, patches of green 

 began to appear in the blackened soil. 

（2）曹老师划横线处的句子如果不是在此处的语境中，而是单独一个句子，我们这么改对吗？因为grow是持续性终结性动词， 持续性终结性动词过去时和完成时是不能跟持续性动词的。

原句：

4、Winter was coming on and the hills threatened the surrounding villages with destruction, for heavy rain would not only wash away the soil but would cause serious floods as well. 

5、the forest authorities ordered several tons of a special type of grass-seed which would grow quickly.

我的第三个疑问：

（1）这里为什么用到了情态动词过去式，是笔者表示委婉的说法，就如同could i come in?

此处依据《情态动词和语气的定义理解》我的理解对吗？

（2）如果把情态动词去掉，谓语动词就用过去式不用过去完成时

Winter was coming on and the hills threatened the surrounding villages with destruction, for heavy rain not only washed away the soil but caused serious floods as well.