Dummy

Because, aside from the verbs to be and to have, only auxiliaries can be inverted to form questions and only auxiliaries can take negation directly, a dummy auxiliary do is used for questions and negatives when only a full verb exists in the positive statement (i.e. there are no auxiliaries in the positive, non-interrogative form). The same dummy do is used for emphasis in the positive statement form. This is known as do-insertion.

For example, if the positive statement form is:

I know the way.
the interrogative, negative and emphatic forms are respectively:

Do you know the way?
I don't know the way.
I do know the way.
Compare this with:

I should know the way.
Should I know the way?
I shouldn't know the way.
(and the emphatic form has to be marked by intonation or punctuation).