how 是副词,强调方式方法,
what是代词,充当do的宾语.
I don't know how to do it=I don't know what to do.
how to do 似乎没有这种说法,一般都说how to do it.意思是怎样做这件事情
what to do 说的是做什么
两个句子都可以做宾语从句
The work is so difficult,I don't know how to do it.工作太困难,我都不知道怎么去做
Summer holiday is so long,and I don't know what to do.暑假太长了,我不知道做什么好.
Suddenly she came,and I don't know what to do.她来得太突然,我不知所措.
Your angel will tell you how to speak.你的天使会告诉你该怎么说话.