当can前面的句子和can一起已经是一个完整的句子，后面跟一个解释或者表意图的短语时。比如Mary likes doing what she can to help others .（大概是这样我也不敢确定😉）
He did what he can to support his family.要从句子结构上看。
can是情态动词 要跟动词原形
