Dấu hiệu:
Once, When, While, As soon as, By the time, Before, During, till, until, As (khi), Since (từ khi), Just as (ngay khi), Whenever (Bất cứ khi nào), No sooner...than...= Hardly/Scarcely ...when...(Vừa mới...thì đã...)
Ex 1: Whenever you are free, we will practice speaking English.
Ex 2:Hardly/Scarcely had she had a shower when the phone rang.
Ex 3: Just as he entered the house, he saw a thief.