They do what they can to help others.为什么不是They do what they can do to help others?

They do what they can to help others.为什么不是They do what they can do to help others?

They do what they can to help others.
他们尽其所能来帮助别人。
They do what they can do to help others?

第二个do关键就是与前面第一个do重复了，在句中就显得累赘！我个人认为其实这句话在早些时候第二do肯定是有的，后来因为口语简便表达的需要逐渐给省略掉了！这个不用深究，记住就可以了！

希望能帮到你

what they can 是do的宾语从句，修饰do。所以不需要再加do导致重复

因为前面已经有动词do 再用do就重复了 故can后面不需再用do

因为do与前面动词重复，所以我们可以把它省略掉了。希望我能帮助你解疑释惑。

do what one can do to do something
加do和不加do都对。习惯上省去do。

They do what they can（do in order） to help others. ------运用了“省略法”，让句子更简练。