once用于什么时态

once作副词意思是：曾经用于过去式。

例如：Once we were good friends.

once 作连词时，可连接两个句子，引导条件状语从句，可用于一般现在时，也可用于一般过去时。例如：Once he makes up his mind ，nothing can not change it.

Once he made up his mind，nothing could not change it.