What are the grammar rules of modal verbs?

This is the third of four chapters about Modal Verbs. To complete this reader, read each chapter carefully and then unlock and complete our materials to check your understanding.   

– Explore four common grammar issues with modal verbs

– Explore three common syntactic issues with modal verbs

– Provide correct and incorrect examples to guide the reader

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Chapter 3

Now that we’ve discussed the nine pure modal verbs and six semi-modals and have explored their importance in an academic context (as well as discussed why students often struggle to use these words in practice), this third chapter on the topic focuses more specifically on the grammar and syntax of each modal verb.


What are the grammatical rules of modal verbs?

While the nine pure modal verbs share many of the same grammatical characteristics, the same cannot be said for the six semi-modals as outlined in Chapter 1. We’ve therefore listed four areas of grammar that learners should focus on below and have highlighted which modal verbs do not follow these simple rules.


1. Inflection

Inflection in the English language occurs mostly on verbs and may be used to demonstrate grammatical properties such as personnumber or tense. However, the general rule to remember here with modal verbs is that this word type is not inflected to agree with the third-person as most main verbs are. While this lack of subject-verb agreement may simplify modals somewhat, some students may in turn overextend this rule and add the third-person ‘-s’ where it isn’t needed

Modal Verbs 3.1 No Inflection
Modal Verbs 3.2 Inflection Error
Modal Verbs 3.3 Third-Person Inflection

2. Pronunciation

As briefly mentioned in Chapter 2, some modal verbs may also demonstrate both strong and weak forms when used in connected speech. Strong forms are most often necessary when the modal verb occurs without a main verb (as in: “Yes, I can.”) or when stressing the verb for particular effect (as in: “I CAN swim.”). In all other instances, the following five modal verbs are pronounced in their weak forms:  

Modal Verbs 3.4 Weak and Strong Pronunciation

3. Tense

Of all the nine pure modal verbs, ‘could’ is the only one that can be used to express past time reference. Because modality tends to deal with either real or hypothetical events, modal verbs are therefore generally used to only refer to the present or future and not the past. While the two semi-modals ‘be able to’ and ‘have to’ are unlike the other modal verbs in that they may be fully inflected for tense, a student may simply use the ‘have’ + ‘past participle’  perfect-aspect construction to emulate the past tense, as in the following two examples:

Modal Verbs 3.5 Present
Modal Verbs 3.6 Past

4. Verb Phrases

Generally in the English language, if one verb precedes another in a verb phrase then that second verb will be formed as a full infinitive using ‘to’, such as ‘to study’ in the following example:

Modal Verbs 3.7 Infinitives

However, as you may have noticed in these examples, modal verbs such as ‘can’ or ‘should’ instead usually precede bare infinitives (which are uninflected main verbs without the word ‘to’ before them). This rule of deleting ‘to’ before the second verb is true for all modal verbs except for the three semi-modals shown below:

Modal Verbs 3.8 Full Infinitives

What are the syntactic rules of modal verbs?

In addition to these four grammar rules, there are three additional rules that are related to how phrases and sentences involving modal verbs are structured.


5. Auxiliary Nature

Because modal verbs are a type of auxiliary (assisting) verb, such verbs must almost always appear in combination with another verb in a verb phrase. The only occasion in which modal verbs may be the only verb within a clause is when there are aspects of that clause that are assumed or have been omitted by the speaker. Such omissions are usually the case when answering questions, as is demonstrated in the examples below (which use brackets to demonstrate the assumed elements):

Modal Verbs 3.9 Auxiliary
Modal Verbs 3.10 Omitted

6. Negation

While most verbs are negated in the English language by placing the words ‘do not’ or the contracted form ‘don’t’ immediately before – as in “I don’t want to study”, this is not usually the case for modal verbs. The various negated versions of each of the fifteen modal verbs have been provided for you in the table below.

Modal Verbs 3.11 Negation and Contraction

Please note two things here, however. Firstly, it’s much more common to use ‘should’ instead of ‘ought’ in negative constructions in modern English as ‘oughtn’t to’ now sounds particularly awkward or ungrammatical to native speakers of the language. Secondly, remember to avoid using the contracted forms of these modals when writing academically, as forms such as ‘can’t’ or ‘needn’t’ are quite informal.


7. Question Forms

Finally, when forming questions using modal verbs, the most common syntactic rule is to move the modal verb before the subject, as in the following example:

Modal Verbs 3.12 Declarative Statements
Modal Verbs 3.13 Interrogative Questions

While this structure may apply to twelve of the fifteen modal verbs we’ve described throughout this short reader, for two of the semi modal verbs (‘had better’ and ‘have to’) the correct constructions when forming questions are as follows:

Modal Verbs 3.14 Had Better and Have To

For ‘ought to’, common practice is to instead use ‘should’ – much like when negating.

3 of 4 Chapters Completed


Once you’ve completed all four chapters about modal verbs, you might also wish to download our beginner, intermediate and advanced worksheets to test your progress or print for your students. These professional PDF worksheets can be easily accessed for only a few Academic Marks.

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Spring 2022