请帮我解释一句英文。关键在为什么这么写。老师说了，但是我没有听懂，，，，麻烦说清楚一点呀？

是定语从句，complaint to sb.把to放到前面也是可以的
其实这句话还可以换成Mr johnson whom we complained to apologised for the mistake
这个主要是介词加关系代词的定语从句，上面那两句话没有下面这句话用的要简练，而且更高级。类似的还有
当介词放在关系代词前面时，关系代词只能用which/whom即“介词+which/whom"且不能省略。但当介词位于末尾时可用that/which/who/whom.作介词的宾语，且可以省略。
例如：
(1) The school (that/which) he once studied in is very famous."that/which"可以省略
= The school in which he once studied is very famous."which”不可省略
(2) Tomorrow I will bring here a magazine (that/which) you asked for.
= Tomorrow I will bring here a magazine for which you asked.
(3) We’ll go to hear the famous singer (whom/that/who) we have often talked about.
= We’ll go to hear the famous singer about whom we have often talked.