once后跟to do还是doing

once doing 一旦做某事 结构：连词once +加动名词，可以看作是once 条件状语从句的省略 功能：相当于once 引导的条件状语从句 例如： Once making a promise,you should try your best to fulfil it. =Once you have made a promise,you should try your best to fulfil it.