英语语法老师 这个句子people must first know how to write letters of th

英语语法
老师 这个句子people must first know how to write letters of the alphabet and learn which sounds they represent
您说learn和to write是并列的不定式?也就是说不是to learn,而是learn?我看到有不带to的不定式的情况?
which sounds引导的从句 作learn的宾语 是宾语从句也就是说和I don't know what I should do差不多的?which引导宾语从句在从句中作represent的宾语,而what则作do的宾语,