An auxiliary verb (also called a helping verb) combines with a main verb to build verb phrases. Primary auxiliaries (be, have, do) help form tenses and voices. Modal auxiliaries (can, could, will, would, shall, should, may, might, must) express ability, possibility, and obligation.
What Is an Auxiliary Verb?
On its own, a main verb like write tells you about an action. But English often needs to say more — when the action happened, whether it is still ongoing, whether it is a question or a negative, who is performing it. This is the job of auxiliary verbs. They combine with main verbs to create the complex meanings that English expresses through its rich verb system.
There are two main classes. Primary auxiliaries — be, have, and do — are grammatical workhorses. They can also be main verbs themselves ("I am tired", "She has a car", "He did a great job"), which is why they are called "primary." Their auxiliary role is when they support another verb: "She is working", "I have finished", "Do you agree?"
Modal auxiliaries are a closed class of verbs that add meanings like ability (can), past ability (could), willingness or future (will/would), permission (may/might), obligation (must/should), and suggestion (shall). Modals always take the bare infinitive (no to) and never add -s for the third person: "She can swim" — never "She cans swim".
One important feature of all auxiliary verbs is that they undergo inversion for question formation: "She is leaving" → "Is she leaving?"; "You have seen it" → "Have you seen it?". When there is no auxiliary in the sentence, do/does/did steps in to carry the question or negative: "She likes coffee" → "Does she like coffee?"
Primary Auxiliaries: Be, Have, Do
| Auxiliary | Use | Example |
|---|---|---|
| be | Continuous tenses | She is reading right now. |
| be | Passive voice | The window was broken by the storm. |
| have | Perfect tenses | I have finished my homework. |
| have | Past perfect | She had left before I arrived. |
| do | Questions (simple tenses) | Do you speak French? |
| do | Negatives (simple tenses) | He doesn't know the answer. |
| do | Emphasis | I do believe you! |
Modal Auxiliary Verbs
| Modal | Primary meaning | Example |
|---|---|---|
| can | Ability / permission | I can swim. Can I open the window? |
| could | Past ability / polite request | She could play piano as a child. |
| will | Future / willingness | I will call you tomorrow. |
| would | Conditional / polite request | Would you like some tea? |
| shall | Offers / suggestions (BrE) | Shall I open the door? |
| should | Advice / obligation | You should see a doctor. |
| may | Possibility / formal permission | It may rain later. |
| might | Weaker possibility | She might come, but I'm not sure. |
| must | Strong obligation / logical deduction | You must wear a seatbelt. He must be tired. |
Common Mistakes
Mistakes to Avoid
She can to swim very well.
She can swim very well. (modal + bare infinitive — no 'to')
He musts leave before noon.
He must leave before noon. (modals never take -s in third person)
Does she can drive?
Can she drive? (modal verbs invert directly — don't add 'do')
I am agree with you.
I agree with you. ('agree' is not a continuous verb; 'am' is not needed here)