Auxiliary Verbs: Rules, Explanation, and Common Mistakes

Primary and Modal Auxiliary Verbs: Rules, Explanation, and Common Mistakes

Auxiliary verbs, also called helping verbs, are used with the main verb to form different tenses, voices, and moods in English. There are two types of auxiliary verbs: Primary Auxiliary Verbs and Modal Auxiliary Verbs. This article explains their rules, differences, common mistakes, and includes exercises to test your understanding.

What Are Primary Auxiliary Verbs?

Primary auxiliary verbs are used to form tenses, voices, and questions. There are three primary auxiliary verbs in English:

1. Be (am, is, are, was, were, being, been)

2. Have (have, has, had, having)

3. Do (do, does, did)

Examples of Primary Auxiliary Verbs

• She is studying for the exam. (Present Continuous Tense)

• They have finished their work. (Present Perfect Tense)

• He does not like spicy food. (Negative sentence with “do”)

What Are Modal Auxiliary Verbs?

Modal auxiliary verbs express necessity, possibility, ability, permission, or obligation. The most common modal verbs are:

• Can, Could → Express ability or possibility

• May, Might → Express possibility or permission

• Shall, Should → Express obligation or future intention

• Will, Would → Express future or hypothetical situations

• Must → Express necessity or strong obligation

Examples of Modal Auxiliary Verbs

• You must wear a seatbelt. (Necessity)

• She can play the piano. (Ability)

• He may arrive late. (Possibility)

• You should study harder. (Advice)

Rules for Using Auxiliary Verbs

1. Primary Auxiliary Verbs Help Form Tenses

Primary auxiliary verbs be, have, and do help form continuous, perfect, and passive voice sentences.

✅ Correct: She has finished her homework. (Present Perfect)
❌ Incorrect: She have finished her homework. (Incorrect agreement – "has" should be used with "she")

2. Modal Auxiliary Verbs Always Come Before the Main Verb

Modal verbs are always followed by the base form of the main verb (without "to").

✅ Correct: He can swim very well.
❌ Incorrect: He can to swim very well. (Remove "to")

3. Modal Verbs Do Not Take "-s" in the Third Person Singular

Unlike regular verbs, modal verbs do not change for third-person singular subjects.

✅ Correct: She can dance beautifully.
❌ Incorrect: She cans dance beautifully. (Modal verbs do not take -s)

4. Modal Verbs Do Not Take "-ed" in the Past Tense

Most modal verbs do not have past forms, except for "could," "would," "might," and "should."

✅ Correct: He could run fast when he was younger. (Past ability)
❌ Incorrect: He caned run fast when he was younger. ("Caned" does not exist in English!)

Common Mistakes:

1. Using "Do" When Not Needed

❌ I do can swim.
✅ I can swim.
Explanation: Modal verbs do not need "do" in positive sentences.

2. Using "To" After a Modal Verb

❌ She must to leave now.
✅ She must leave now.
Explanation: Modal verbs are followed by the base verb without "to".

3. Incorrect Use of "Have" in Perfect Tenses

❌ He have gone to the market.
✅ He has gone to the market.
Explanation: "Has" should be used with singular subjects like "he" or "she".

4. Using the "-ing" Form After a Modal Verb

❌ You should studying more.
✅ You should study more.
Explanation: Modal verbs must be followed by the base form of the verb.

Exercises:

Exercise 1: Identify the Auxiliary Verbs

Find the auxiliary verb(s) in each sentence.

1. She has completed her project.

2. They will arrive soon.

3. I am going to the store.

4. He should apologize for his mistake.

5. We do not like horror movies.

Exercise 2: Correct the Mistakes

Rewrite the sentences correctly.

1. She must to leave early.

2. He can plays the guitar well.

3. They has finished their work.

4. We does not want any trouble.

5. I am go to the gym later.

Exercise 3: Choose the Correct Form

1. She (has / have) been to Paris twice.

2. They (do / does) not like spicy food.

3. You (must / must to) be on time.

4. He (can / cans) drive a car.

5. We (should / should to) respect our elders.

Exercise 4: Fill in the Blanks with a Primary or Modal Auxiliary Verb

1. She ________ watching a movie right now.

2. They ________ left before we arrived.

3. He ________ swim very well.

4. You ________ apologize for being late.

5. I ________ not know the answer.