To express probabilities, obligations, habits, and more, we often use words like 'must', 'can’t', 'should', and 'might'.
These modal auxiliaries can be found just about anywhere and have multiple applications,
but they also play by their own rules.
Here are Ten Things to Know about Modal Verbs.
ONE
There are only nine principal modals.
'must' has strong implication, 'shall' & 'should' are moderate, and 'may' & 'might' are weak,
while 'can' & 'could' cover every potential.
Meanwhile, 'will' & 'would' are more neutral, almost matter-of-fact.
TWO
Eight of the nine principal modals come in pairs.
The two words of a pair have similar meanings,
and we often switch from the first to the second when backshifting, as we do in Reported Speech.
Other than that, the difference between the two words in a pair depends on the modal flavor.
THREE
Each modal has multiple meanings.
They can be Modals of Obligation, or Modals of Deduction, Modals of Ability, Modals of Advice, ...
of Possibility, of Habit, Regret, Expectation, Prohibition, and more.
All of these can be sorted into three categories, or flavors:
Modals of Speculation express the speaker’s beliefs and conclusions,
including assessments of Possibility and Probability.
Modals of Regulation express ideals, whether they belong to the speaker, or the speaker is just passing them along.
They include directives of Permission and Obligation.
Modals of Function express the subject’s capability and inclinations,
and include Abilities and Habits.
You probably don’t need to remember all the terms, but
knowing which applications are grouped together helps us understand the various grammar rules.
FOUR
To make Modals of Speculation and Modals of Regulation past, we add ‘have’ after the modal.
Make sure that whichever verb follows ‘have’ is in the Perfect Participle form.
FIVE
Regardless of tense, the verb that immediately follows the modal –
which is often the main verb of the clause, but not always –
needs to be in its base form;
it should not have an -ing, -ed, or -s suffix or undergo any other changes.
SIX
The modal itself should not undergo any changes to its form under any circumstance.
We would never say ‘coulded’, ‘musting’, ‘shoulds’, or anything of the sort,
nor should modals ever be preceded by the word ‘to’.
While all other verbs inflect depending on the auxiliary verb that comes before it or on the subject,
modals should only ever be in their base form.
Because of this discrepancy with every other verb, modals are called ‘defective’.
SEVEN
We can replace defective modals so that they’re easier to work with, grammatically.
The appropriate replacement phrase depends on the flavor, but most come in the form of ‘be [blank] to’.
These replacements allow us to do things like express the past tense with more clarity
or effectively place two modal meanings back-to-back.
EIGHT
In addition to the nine principal modals, there are a handful of ‘semi-modals’ or ‘pseudo-modals’ –
words or phrases that serve the same or similar purposes as modal verbs
and that have some, but not all, of those modal properties.
For instance, phrases like ‘ought to’ and ‘had better’ are defective.
You may or may not study these phrases alongside the principal modals.
NINE
Modal verbs reflect the mood of a sentence
and often indicate an Unreal Mood, which expresses anything short of accepted absolute reality.
As we’ve already seen, some modals communicate what the speaker believes might be true –
as opposed to what is confirmed or generally assumed to be true.
We apply such modals to Future Predictions, Deductive Reasoning, Conditional Sentences, and other hypothetical situations.
Other modals communicate desires and regulations for how things should be, as opposed to how things actually are.
We apply such modals to Future Plans, Requests, Wishes & Regrets,
more Conditional Sentences, and other expressions of standards or restrictions.
These are just some of the many applications of modal verbs.
TEN
Modal verbs are a subset of auxiliary verbs, and act as such.
They can be bundled with 'not';
come before subjects to form questions;
represent the rest of the verb phrase in short answers …
and in question tags;
and otherwise do anything that auxiliary verbs can do.
This has been Ten Things to Know about Modal Verbs.