18.He leaves one hour earlier lest he will miss the train .
19.As he grew up,he got more knowledge.
20.Whoever told you this secret will be punished.
21.In spite of his wealth,he is not happy
22.What the boy needs to know is an understanding of the value of a dollar.
23.Fast as he runs,he did not win the game last time.
24.This apartment consists of two bed-rooms,one bathroom and a kitchen
25.No students will be allowed to be absent without any excuse.
理由：
18.典型的主将从现句型,lest 相当于in case,以防...
19.由as引导的时间状语从句,as 译作"随着...'表示随着一种情况的变化发展,另一种情况也在变化发展.
20.Whoever told you the secret 是一个主语从句.从句本身缺少主语,所以要选用连接代词whoever.
21.despite=in spite of,可引导让步状语从句,译作：不管...;虽然.
22.What the boy needs to know是一个主语从句,连接代词what 在从句中充当know的宾语.A、B 选项的问题在于缺少引导从句的引导词；as不是代词,不能作宾语.
23.as引导让步状语从句表示“即使,尽管”的含义时,不能置于句首,而是要将句子中相应的副词、动词、形容词或不带冠词的名词置于句首.
24.consist in 译作“在于...”consist of 译作“由...组成”本身带有被动含义,不能用于被动语态.
25.put up 译作“张贴；搭建；为...提供食宿”,put off 译作“推迟”,see...off 译作“给...送行”,put in 译作 “放入；提交”.
