Modal Verbs fall into three different categories based on meaning. As a teacher, it helps to be aware of these categories because they 1) allow for different uses of the same word; 2) dictate how multiple modals can be combined in a verb string; and 3) determine whether defective forms can/should be used (we’ll explain this idea later in case you’re unfamiliar with it).
The coursebooks you use may have topics like “Modals of Permission”, “Modals of Deduction,” etc., which are subtypes within the 3 main categories. So if you’re already familiar with those, you’ve got a let up on this post.
The 3 Categories
The three categories of modals are Epistemic (relating to knowledge), Deontic (relating to ideals), and Dynamic (relating to performance).
- Modals of Possibility: can, could, may, might
- Modals of Deduction: could, may, must
- Modals of Expectation: shall, should (rarely), will, would
- Modals of Permission: can, could, may
- Modals of Obligation: must, shall, should, will (imperative)
- Modals of Ability: can, could
- Modals of Habit: might, would
Why This Matters
We won’t go into the intricacies of modal verbs having three categories, as teachers wouldn’t find much practicality in those depths for most situations. But in case you’re wondering how to address what seems like inconsistencies with modals, being aware of the three categories might help. Here’s just a glimpse.
MULTIPLE USES PER WORD
Take ‘can’ as an example: “Can this really be happening?” (EPISTEMIC); “Can I use the restroom, please?” (DEONTIC); “I can do thirty push-ups.” (DYNAMIC).
These commonly-used words having multiple meanings can be tricky for students to wrap their head around. But the good news is that in most circumstances, only one category is applicable, so you only need to consider one definition for each modal.
COMBINATION & ORDER
We rarely use more than one modal at a time, but you can use two in the same verb string if they’re from different categories. You can use an Epistemic then a Deontic, or you could use an Epistemic then a Dynamic.
I once knew someone who would often say “We might could do that.” That was normal for a Southern U.S. dialect, but more widely we would use a defective form for the second modal in the pair, so instead I would say “We might be able to do that.” Consider other possibilities like “She might need to go there,” or “He must be able to do it on his own.”
DEFECTIVE & PRETERITE FORMS
Defective forms are multi-word modals, most of which start with ‘be’ and almost all of which end with ‘to’. For example, ‘be able to’ is the defective form of can/could, and ‘be going to’ is the defective of will.
Must has multiple defectives — ‘have to’, ‘have got to’, and ‘need to’ — each of which is an exception since they don’t start with ‘be’. But all three of those defective forms apply to must as Deontic (permission). Must as Epistemic (deduction) curiously has no defective form.
If every you’re wondering why a modal acts one way in certain instances and a second way in other instances, see which category that word falls into for each case, and hopefully that’ll help answer your question!