Ex. I don't know what to do. = I don't know what I should do.
Show them how to do it. = Show them how they can do it. or Show them how you can do it.
As far as I know, the subject of 'interrogative+to-infinitive' has the same reference with the previous noun depending on the context. Then, if someone asks me
Ex. Do you know how to get to the airport?
this can only be understood as 'Do you know how you can get to the airport?'. (I instead of you is unlikely.)
Do I understand it correctly?
cho7712 Ex. Do you know how to get to the airport? '
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cho7712Ex. Do you know how to get to the airport?
This is understood as 'Do you know how anyone can get to the airport?'