解析：do是及物動(dòng)詞,所以后面接的是賓語(yǔ)從句.【what they could 】【to】可以分為兩部分來理解what they could（do）其實(shí)這里省略了動(dòng)詞【do】,what 做do的賓語(yǔ) .what they could do可以理解成一個(gè)名詞.就是做前面那...真不是很理解，could不是情態(tài)動(dòng)詞嗎，加動(dòng)詞原形？對(duì)啊 ，could是情態(tài)動(dòng)詞，后面接動(dòng)詞原形，上面不是說其實(shí)could后面還有do省略了。what they could do盡他們所能。這是賓語(yǔ)從句。what 做do的賓語(yǔ)。你就把 what they could do當(dāng)成一個(gè)名詞就行,就用sth來代替。They tried their best to do 【sth】to help us不理解的話再詳細(xì)點(diǎn)問再詳細(xì)點(diǎn)吧，灰常感謝！！【what】賓語(yǔ)【 they】主語(yǔ)【could （do）】謂語(yǔ)，可以顛倒下語(yǔ)序。They could do what to。。。。to 表示目的。再不明白我也沒辦法了你仔細(xì)看看回答吧