american english - Differences between "how to do something....." and "how do I do something..."

"how to...." is correct ? or Impolite？
I'm not really make sure What's the difference between the two.

Answer

As someone already mentioned, "how to do" can only be a part of a statement. For example,

"I don't know how to do it."

So if you intend to ask a person how something should be done, you should say,

"How do I do it?" and not "How to do it?"

This isn't impolite but if you want to be more polite, try

"Could you please tell me how to do it?"