somestudentsfeelthepressuretodowellintheirlessons1这里一定要用todo吗,请问todo后面做什么结构,后置定语吗,能不能用doing做定语,区别是?什么时候用不定式做定语；2as表倍数时使用方法,比如twice

Question

some students feel the pressure to do well in their lessons
1这里一定要用to do 吗,请问to do后面做什么结构,后置定语吗,能不能用doing做定语,区别是?什么时候用不定式做定语；
2as 表倍数时使用方法,比如twice the risk as they?对吗,只有一个as的,表什么方面的几倍怎么说

Answer 1

1、
不定式作后置定语,修饰pressure；不能直接用doing作定语,因为feel后面接的宾语pressure与doing没有逻辑上的主谓或动宾关系,不是pressure doing也不是doing the pressure.
例如：
I saw him doing his homework when I passed his window.
主谓关系,him和doing存在逻辑上的主谓关系,是him doing
We all found his equipment interesting. (主表关系)
I saw Mary going upstairs then. (主谓关系)

以上例句,都是现在分词在句中作宾语补足语,这时现在分词和前面的宾语有逻辑上的关系
现在分词短语作【定语】放在所修饰词的后面,意思同【定语从句】差不多.
例如 ：
Tell the children playing outside not to make too much noise. = Tell the children who are playing outside not to make too much noise.
关于区别：
动词不定式常表示未来的动作,所以叫不定式.动名词则表示经常性或已经发生的动作
例如：
He stopped to do speak to me.（未发生,将要发生）
He stopped speaking to me.(已发生）

什么时候用不定式作定语：
一句话说不清,你看下这里,也许会有点帮助：
http://wenku.baidu.com/view/2c255d4fe518964bcf847ccc.html

2、twice the risk as they是不对的,前后比较的不一致
你可以说：We have twice the risk as they do.

as 用来表示倍数,有下列几种用法：
A is 倍数＋as +形容词原级＋as B
A is 倍数＋the size/width/height/length of B
例如：
His uncle is twice as old as he.
The bag is twice the size of that one.
注意：A和B 须是同等成分,如果A是名词,B也同样要是；其他,亦成立!