After the students came to the lab, the professor brought out three bottles.

　　学生们到实验室后，教授拿出了3个瓶子。

　　3．表示"持续性"和"瞬间性"的连词主要包括since, ever since, till/until等。

　　(1)since, ever since引导时间状语从句时，从句中多用一般过去时，而主句中谓语动词多用现在完成时。

　　I have lived heresince I came here.

　　自从我来到这儿，我一直生活在这儿。

　　(2)till和until(till不可以位于句首)：在肯定句中，主句的谓语动词是延续性动词，而在否定句中，主句的谓语动词是瞬间性动词。

　　He waited until/tillhis mother came back.

　　他一直等到他妈妈回来。

　　I didn't go to bed until/tillI finished my homework.

　　直到做完作业我才上床睡觉。

　　[即时演练1]

　　(1)用适当的连词填空

　　①It will not be long before he realizes his dream.

　　②It is five years since I last met him.

　　③After the children had gone to bed, she began to prepare her lessons.

　　④We won't start our discussion until/till he comes.

　　⑤When I got home, my parents had fallen asleep.

　　⑥While we were talking, the teacher came in.

　　⑦As time goes by, we have a better understanding of things around us.

　　(2)完成句子

　　①It is three years since_he_came_to_our_school.

　　他来我们学校已经三年了。

　　②It_won't_be_long_before we have the midterm examination.

　　过不了多久我们就要期中考试了。

　　③Mum didn't_go_to_bed_until I came back last night.

　　昨晚直到我回来妈妈才上床睡觉。

　　4．表示"一......就......"的连词有：as soon as, the moment, immediately, directly, no sooner ... than, hardly ...when等。

　　I told him the news the moment I saw him.

　　我一看见他就把那个消息告诉他了。

No sooner had he got to the station than the bus left.