It is known to us 是主句,how he became a writer.是从句.
要辨别主句和从句有两种办法：
1.理解意思
2.从句是修饰前面的主句,从句是：疑问词+陈述句.（比如"how "就是疑问词,“he became a writer”就是陈述句） .剩下的理所当然的是主句.