Grammar for Dummies

Although I haven't read it yet, The Grammar Bible by Michael Strumpf and Auriel Douglas sounds like my kind of book. It even contains a section on modal auxiliary verbs, those bugaboos of English learners. A quick study of the first 20 or so returns from Google reveals the most-used resources for modal auxiliary verbs are academic and often United Kingdom universities. Hmm. I consulted a more global and understandable source Wikipedia.

"The Wik" says:  Auxiliary verbs are, loosely, verbs that specify more about the main verb that follows. For English auxiliary verbs, the specified 'more' alters the basic form of the main verb to have one or more of the following functions: Passive, Progressive, Perfective, Modal, and Dummy.
 
Leaving the Dummy function to those who write books "for Dummies", I scrolled down to read:

There are nine modal verbs: can/could, may/might, shall/should, will/would and must. They differ from the other auxiliaries both in that they are defective verbs, and in that they can never function as main verbs. They express the speaker's (or listener's) judgement or opinion at the moment of speaking. Some of the modal verbs have been seen as a conditional tense form in English.
 
As The Grammar Bible authors put it, how to use modal auxiliaries is "confusing". I would write more on this if I could, but I might make a mistake. I must end it here. I shall.