In English, auxiliary verbs (have and be) and modal verbs behave differently from all other verbs. For example, they combine differently with negatives, as shown in the following table for auxiliaries (have and be), a modal verb (can) and another verb (go).
|Aux (have)||You have gone||You have not gone|
|Aux (be)||You are going||You are not going|
|Modal||You can go||You cannot go|
|All other||You go||You do not go|
For the auxiliary and modal verbs, the negative word not is inserted after the auxiliary or modal verb (and before the main verb, gone / going / go in this example). For all other verbs, not is inserted before the main verb, but a dummy auxiliary verb (do) must be inserted before not.
The English verb must is clearly a modal verb, but its near synonym need to is only marginally a modal verb. It can sometimes behave like either a modal verb and sometimes like a main (non-modal) verb. For example, two different constructions can be used to form a negative:
- You need not run [like a modal verb]
- You do not need to run [like a main verb]
The modal verb construction (need not) is perhaps slightly more elegant, though maybe becoming a little old fashioned (archaic). But non-native speakers of English often find it harder to understand than the normal main verb style (does not need to). So if I’m writing for non-native speakers, I now use only the main verb version (does not need to) and not the modal version (need not).