A question word often connects statements containing the verbs know, understand, wonder, ask, and tell with a noun clause (subject + verb combination).

introduction     +    question word  +    noun clause

I know                                                    

I don't know      

Do you know           who                        he is.(?)

I understand                                           

I wonder                                                 

Ask him                                                 

Tell us                                                    

    Basic Patterns for Prepositions in Noun Clauses:      
 

    A. CLAUSES WITH BE     

introduction     +    question           +    noun clause

                              word                      subject       +    verb     +    preposition

I know                     who(m)                   the letter           is                to.

I know                     what                       the letter           is                about.

I know                     where                      the letter           is                from.

I know                     what state               the letter           is                from.

I know                     which state              the letter           is                from.

    B. CLAUSES WITH OTHER VERBS     

introduction     +    question          +    noun clause

                              word                      subject   +    verb (object)    +    preposition

I know                     who(m)                   she               writes letters           to.

I know                     what                       she               asks                       for.

I know                     where                     she               writes                     from.

I know                     what state              she               writes                     from.

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