Question

【英语】急求一道英语题解释

语法填空 (共10小题，每小题1.5分，满分15分)
Viennese-born composer Frederick Loewe, 16 we remember from his classical musicals including “My Fair Lady” and “Camelot”, was not always famous. He studied piano with the great masters of Europe and achieved huge success as a 17 (music) and composer in his early years. But 18 he immigrated to the United States, he failed as a pianist. For a while he tried other types of work including gold mining and boxing. 19 he never gave up his dream and continued to play the piano and write music.
20 those difficult years, he could not always afford 21 (make) payments on his piano. One day, bent over the keyboard, he could hear nothing but the music that he played with such rare inspiration. When he finished and looked up, he was astonished to find that he had 22
audience of three moving men who 23 (seat) on the floor.
They said nothing and made no movement toward the piano. Instead, they dug into their pockets, pooled together enough money for the payment, placed it on the piano and walked out, empty 24 (hand). Moved by the beauty of his music, these men recognized excellence and responded to 25 .
问题在第18个空处，请帮忙解答为什么不能用after，只能用when？答案为when

Answer 1

如果用after，则其引导的从句谓语动词用过去完成时（移民在前，当钢琴家在后）

追问

网上及老师的解释如下：由于after本身具有“在…以后”的意思，after引导的时间状语从句中常用一般过去时代替过去完成时；after引导的动名词分句中常用一般式代替完成式。
那为什么这里不能用after

追答

在新概念2第18课，有关于after关于过去完成时的用法，至于这道题用排除法做就行了

Answer 2

如果是after只能用完成试了，而when是地地道道的引导时间状语的词，谢谢

Answer 3

时间状语只能用WHEN引导