As the plane circled over the airport,everyone sensed that s

As the plane circled over the airport,everyone sensed that something was wrong.The plane was moving____16____(steady)through the air,and although the passengers ___17___(fasten) their seat belts,they were suddenly thrown forward.At that moment,the air-hostess presented.She looked very pale,__18___ was quite calm.Speaking quickly and almost in a whisper,she informed everyone that the pilot had fainted and asked if ___19___ of the passengers knew anything about machines,After a moment‟s hesitation,___20___ man got up and followed the hostess into the pilot‟s cabin.Moving the pilot aside,the man took his seat and listened carefully to the urgent instructions that ___21___(send) by radio from the airport below.To everyone‟s relief,the plane,__22__ was dangerously close to the ground at the moment ,soon began to climb.The man had to circle the airport several times to become familiar ___23___ the controls of the plane.The critical moment came ___23___ he had to land.The man,____25____(follow) the instructions,guided the plane toward the airfield,and it landed safely after a long run along the runway.
unsteadily; 17.had fastened; 18.but; 19.any; 20.a; 21.were being sent; 22.which; 23.with; 24.when; 25.following
英语不是很好，所以请解释的清楚一点，把相关的语法点也讲一下，

16 修饰动词move的进行时态,要用副词steadily
17 因为系安全带是在他们被扔出去之前,是在过去的过去,故用 had fastened
18 but 表转折,虽然脸色很苍白,但是依然很镇定.
19 any 表示“任何”,这里的 if 不是条件句里面的 if ,它的意思是“是否”.
20 a 表示泛指,泛指一个男人.
21 instructions 和 send 存在被动关系,且强调当时收音机正在播放的,所以用过去进行时的被动语态.
22 which 引导的非限制定语从句,指的是plane,并在定语从句充当主语,即是 is dangerously close to the ground 的主语.
23 be familiar with 对...很熟悉
24正常的语序是：the critical moment when he had to land came.,只因主语过长,when 后面的句子才后置,when he had to land 是做the critical moment 的后置定语里的时间状语成分.
25 follow 和 the man 存在主动关系,这里是现在分词做伴随状语,表示和guided这个动作几乎同时发生的一个动作 .