NOUN STRUCTURE- whatever can function as a noun

          * As objects:

                              They know how to say hello.

                              They know about that.

                              They know the meaning of saying hello.

                              They know why they say hello.

                              They want  to say hello.

*As complements:

                   I tell  them   how to say hello.

                   I tell  them   about that.

                   I tell them    the meaning of saying hello.

                   I tell them    why they say hello.

                   I tell them    to say hello.