how不能,但what可以
1 What was impossible 50 years ago may be quite possible today (这是what引导的定语从句)
== Something that was impossible 50 years ago may be quite possible today.（定语从句）
2 How she managed to finish her hard work on time is beyond my ken.（主语从句）
== The way that / in which she finished her hard work is beyond my ken. （定语从句）
3 She told them what she had seen there （定语从句）
== She told them all / everything that she had seen there.（定语从句）
4 They are building a second library in what was once the school cinema..（定语从句）
== They are building a second library in the house that / which was once the school cinema. ..（定语从句）