Question

They tried their best to do __________ help us. A. that they could B) all that they could

They tried their best to do __________ help us. A. that they could
B) all that they could C. what they could to D. what they could
请高人解释下为什么是C呢？

Answer 1

解析：do是及物动词，所以后面接的是宾语从句。

【what they could 】【to】可以分为两部分来理解

what they could（do）其实这里省略了动词【do】，what 做do的宾语 。what they could do可以理解成一个名词。就是做前面那个do的宾语。

动词不定式【to】表示目的，为了、、、

下面逐一分析各个选项：

A、that 引导宾语从句的时候，只起到连接的作用，不做句子成分，如刚才所说。could后面省略了do，所以do要接宾语，that不符合。批出A
B、all that=what，但是选项中缺少to做目的
D 同样缺少to 不赘言。
如有疑问，请追问。

无疑后请采纳。

书山有路勤为径，学海无涯苦作舟。

-----------与君共勉-----------

追问

真不是很理解，could不是情态动词吗，加动词原形？

追答

对啊 ，could是情态动词，后面接动词原形，上面不是说其实could后面还有do省略了。
what they could do 尽他们所能。这是宾语从句。what 做do的宾语。你就把 what they could do当成一个名词就行,就用sth来代替。
They tried their best to do 【sth】to help us
不理解的话再详细点问

追问

再详细点吧，灰常感谢！！

追答

【what】宾语
【 they】主语
【could （do）】谓语，可以颠倒下语序。
They could do what to。。。。to 表示目的。
再不明白我也没办法了
你仔细看看回答吧

Answer 2

因为do sth. to help sb.
help之前需要to

Answer 3

what they could 相当于 sth 做do 的并与从句，to do表目的，do sth to help us.
希望对你有所帮助，

Answer 4

what they could是做do的宾语，to help us是做目的状语。