They who can not do as they would,must do as they can.分析句子结构

第一个they是主语,who can not do as they would作为第一个they的定语从句用来修饰第一个they,
就是说 they who can not do as they would 整一句作为一个主语
do是谓语
as they can 作为宾语
整句意思为：不能之人也须尽其所能.