The auxiliary verb 'to do' (Present he/she/it 'does'; Past 'did') helps form questions and negative sentences : What did you expect? I don't really remember.
Modal verbs form complex predicates with an infinitive: 'to have to', need to (necessity), to be to, must (obligation), can, could (possibility), may, might (permission), shall, should (recommendation), would (supposition).
Example:I have to do the assignment by tomorrow. His opinion is to be taken seriously. You may come in. He might as well be at home. Can/could you pass the salt? The work must be done on time. She needs to check her schedule. Shall I turn on the light? They shouldn't do it. I would take this position if I were you. Of all verbs only 'need' and 'to have to' require use of the auxiliary 'to do' to form questions and negative sentences : Do you have/need to go there? I doseen't have/need to.
Other verbs form questions and negations by themselves: Have you ever been to London? What was he reading when you came? Shouldn't I do it? It hasn't been done yet. We won't do anything.