Confused about Auxiliary and Modal Verbs? Simple Guide to Functions and Rules

Contents

In this article, we’ll look at the following points:

What auxiliary verbs are and how they function within a sentence.
The difference between auxiliary verbs, primary auxiliary and modal auxiliary verbs in meaning and usage.
The main rules for using them correctly in English.

Quick Summary

Auxiliary verbs are helping verbs that work with the main verb to express a complete idea. They can be divided into two main types:

Primary auxiliary verbs (be, have, do)
Modal auxiliary verbs (can, may, must, etc.)

The primary auxiliaries help form grammatical structures, such as tense, aspect, and voice, while the modal auxiliaries add meanings like possibility, obligation, or probability to the main verb.

When using a modal verb, a sentence generally follows this structure:

Subject + modal verb + base form of the main verb (+ object)

Always remember that the verb after a modal must be in its base form.

For example,

Correct: I can play the piano.
Incorrect: I can played the piano.

The second sentence is wrong because the past-tense verb played follows the modal verb can. Modal verbs such as can, will, must, may, and should are always followed by the base form of a verb, not its past or -ing form.

Two Types of Auxiliary Verbs

Auxiliary verbs are divided into two main types:

Primary auxiliary verbs (be, have, do)
Modal auxiliary verbs (can, may, must, etc.)

When we refer to “auxiliary verbs,” we generally mean one of these two categories.

Simply put, primary auxiliary verbs help form grammatical structures such as tense, aspect, and voice. For example, we change the form of ”be” to indicate different tenses, past, present, or perfect.

Modal auxiliary verbs, on the other hand, add specific meanings to the main verb. Depending on which modal verb is used, the sentence can express possibility, obligation, ability, or probability.

She can speak three languages.

Here, the modal verb ”can” modifies the main verb ”speak” to emphasize the ability to perform the action. Now, let’s look at more detail of these two types of auxiliary verbs.

Primary Auxiliary Verbs

Primary auxiliary verbs are essential for forming grammatical structures such as tense, aspect, and voice. This group consists of the following three verbs:

be
have
do

Unlike modal auxiliary verbs, these verbs don’t carry a clear meaning by themselves. Instead, they serve a grammatical purpose, helping the main verb show when an action happens or how it’s expressed.

For example, it’s difficult to assign a specific meaning to “be.” It doesn’t describe an action like run or speak does. However, it plays an important grammatical role, such as forming the progressive tense (She is studying) or the passive voice (The book was written by Tolstoy).

In contrast, modal verbs like can or must add a clear nuance to the main verb, expressing ability, necessity, or possibility.

Modal Auxiliary Verbs

Modal auxiliary verbs work together with the main verb to add extra meaning. They can express possibility, necessity, ability, or time, depending on the type of auxiliary verb used. Let’s compare two sentences: one without an auxiliary verb, and one with it.

She plays the piano.

This sentence simply states a fact. It tells us that she plays the piano. That’s all. It doesn’t specifically highlight ability or permission; it just presents an action as a general truth.

She can play the piano.

In this version, the auxiliary verb “can” changes the meaning by adding a sense of ability. Now, the focus is not just on the action itself but on her capability to perform it.

Here are example sentences using an modal auxiliary verb.

My sister should study more to pass the exam.

Subject: My sister
Aux verb: should
Main verb: study
to-infinitive phrase: to pass the exam

In this example, the auxiliary verb should expresses a sense of necessity or advice, and it is directly followed by the base form “study.”

Common Modal Auxiliary Verbs and Their Meanings

Modal Verb	Core Meaning / Function	Example Sentence
can	ability or possibility	She can speak three languages.
could	past ability or polite possibility	When I was a child, I could swim very well.
may	permission or possibility	You may leave early today.
might	weaker possibility	It might rain later.
must	strong obligation or logical conclusion	You must wear a seatbelt. / He must be tired.
shall (formal)	future or determination (mainly in formal English)	We shall overcome the problem.
should	advice, mild obligation, or expectation	You should study harder.
will	future intention or certainty	I will call you tomorrow.
would	hypothetical or polite request	I would go if I had time. / Would you like some tea?

Summary Table (Primary vs. Modal Auxiliary Verbs)

Type	Examples	Main Function	Example Sentence
Primary Auxiliary Verbs	be, have, do	Used to form grammatical structures such as tense, aspect, and voice.	She is studying for her exam. (progressive tense)
Modal Auxiliary Verbs	can, may, must, will, should, could, might	Used to add meaning to the main verb, expressing ability, necessity, or possibility.	You must finish this task. (obligation)

About the Author: joe

Hi, I’m Joe. I completed my BA and MA at Purdue University. My research topic was effectiveness and reliability of standardized tests like the SAT and TOEFL. After that, I worked in the test development industry and designed questions to make sure they reflected the skills of interest. I’ve spent time on both the research side and the practical side of test design, and I want to support SAT learners here. Having seen how test questions are actually made, I can give students effective strategies for SAT test takers.