【问】 连词before与连词until都表示主句的情景发生在从句的情景之前，那末它们有没有区别呢?
 【答】 在用法上主要有两个区别：第一，带有until从句的肯定式主句中的谓语动词必须是持续性的，而带有before从句的主句中的谓语动词，则不必是持续性的。试比较：
 1. I waited until he returned.
 我一直等到他回来。
 2. I shaved before I went to the party. (CGEL)
 在参加宴会之前，我刮了胡子。
 类例：
 1. We always have to wait until the last customer has left.(LEG)
 我们总是得等到最后一个顾客离去。
 2. They will live in Chicago until William finishes his thesis. (CGEL)
 他们在芝加哥将一直住到威廉写完论文。
 3. The performance lasted until the actors were exhausted. (CGEL)
 演出一直持续到演员们个个精疲力尽。
 第二，until引出的分句的内容是真实的，而before引出的分句的内容可能是真实的，也可能是非真实的。
 1. Let's wait until the rain stops. (Palmer)
 让我们等到雨停吧。
 2. And before she could speak again I began to talk. (Wells)
 她尚末再开口，我就开始说话了。
 3. Pigs will fly before he'll become a mathematician. (CGEL)
 他要是能当数学家，猪都会飞了。
 请做下面英语试题：
 1. According to scientists，dinosaurs dominated the earth for 150 million years____theysuddenly vanished 65 million years ago.
 A. unless B. until C. when D. then
 2. ____in 1788，the thirteen original states worked together under an agreement called
 the Articles of Confederation.
 A. Not until the Constitution was adopted
 B. The Constitution was adopted
 C. Adopted by the Constitution
 D. Until the Constitution was adopted
 3. I will try to finish this work__.A. up until you can come B. before you can come
 C. up to the time you are able to come D. until you can come (TOEFL，80. 1)