Interesting question with a complex answer, as Felipe suggested, sadbird. From a grammatical perspective, it helps if you treat need primarily as a lexical verb (what you call a "main" verb) but one which can also express modality in much the same way as other modal verbs (which is why it�s often referred to as a semi- or quasi-modal).
As a purely lexical verb (that is, it has different forms, for example, which convey tense, aspect and voice), need takes -s in 3rd person singular, has -ed and -ing inflections for past and participial forms, requires auxiliary do for interrogatives and negatives, and is normally followed by a to-infinitive. Just like a �normal� verb:
–Everybody needs somebody sometime.
–Wow! I needed that.
–Do you need a hand?
–I don�t need this!
–They need to study harder.
As a modal, it would have the same behaviour as other modal verbs (no special inflection for tense etc, no need for auxiliary do, never occurs without a main verb except when implied), but is usually followed only by a bare infinitive. This is where it gets interesting, though. Modal need is most often used in negative statements to express a lack of necessity ("You needn�t sit the exam if you don�t want to"), although it turns up – very rarely – in questions ("Need I go on?"), and ever so rarely in whether- and if-clauses ("I doubt whether anyone need do this"). You�ll probably find that many grammar books will say that these forms are less common in American English.
As far as integrating all this into a lesson, you could do worse than find a contemporary text where modality – and particularly elements like necessity, duty and obligation – is used extensively. The current pandemic provides myriad examples from official documents and instructions to press articles of the various shades of modality and how they might be employed, depending on context (lots of emphasis on permission, possibility, obligation – and, of course, necessity). This, for example, is an article (from the US) which uses variations of need at least half a dozen times (including the semi-modal in the headline):
