learn 是个及物动词,后面要直接接宾语,learn something,这里的宾语由how引导的短句充当.疑问词how是询问方式方法,怎样保护我们自己.
疑问词（who,what,which,when,where,how）+不定式在句子中可以做主语,例如：When to start has not been decided.何时动身尚未决定.作宾语：
I don't know what to do .我不知道该怎么办.做表语：The question was where to go.问题在于到哪儿去.做双重宾语：I asked him how to learn English.我问他如何学习英语.具体选择哪个疑问词要看是对什么提问.