这是错句.应该是trying to catch that thing which hurt my boy.
这是一句典型的定语从句.先行词是that thing,引导词用which在从句中代替先行词做主语.
有what的从句,要看what是否有疑问含义.如果有就是名词性从句.
如果没有就是定语从句.当引导定语从句时,没有先行词,这很特殊.
what you need is more practices.无疑问意义,失去先行词的定语从句.
I know what you want.无疑问意义.失去先行词的定语从句.
I don't know what you want.有疑问意义宾语从句.