An auxiliary verb (or “helping verb”) works together with a main verb to build tenses, ask questions, make negatives, and express ideas such as ability, permission, and obligation. English has two groups: the primary auxiliaries (be, have, do), which help form tenses, and the modal auxiliaries (such as can, must, and should), which add shades of meaning, as set out in the Cambridge Grammar reference. For example, in She has finished her homework, has is the auxiliary that builds the present perfect; in Does he like coffee?, does forms the question. A common mistake is treating a modal like a primary auxiliary, for instance writing She is can swim instead of the correct She can swim. At B1 level, controlling auxiliaries is the key to forming accurate questions and negatives.
Primary Auxiliaries: be, have, do
These three verbs can act as main verbs on their own, but as auxiliaries they help a main verb form a tense or structure.
| Auxiliary | Main use | Example |
|---|---|---|
| be | continuous tenses & passive | She is working. The car was repaired. |
| have | perfect tenses | They have finished. I had eaten. |
| do | questions, negatives & emphasis (present/past simple) | Do you know? I don't agree. I do like it. |
Modal Auxiliaries
Modal verbs come before the bare infinitive (the verb with no to) and never change their form. They express meanings such as ability, possibility, permission, and obligation.
| Modal | Main meaning | Example |
|---|---|---|
| can / could | ability, possibility, permission | I can swim. Could you help? |
| will / would | future, willingness, polite requests | I will call you. Would you mind? |
| shall / should | suggestions, advice, obligation | Shall we go? You should rest. |
| may / might | permission, possibility | May I come in? It might rain. |
| must | strong obligation, certainty | You must stop. She must be tired. |
Semi-Modals
Some expressions behave like modals in meaning but follow different grammar rules. These are called semi-modals or modal phrases.
- have to — obligation (often from outside rules): I have to wear a uniform.
- need to — necessity: You need to sign here.
- ought to — advice, like should: You ought to apologise.
- used to — past habits no longer true: I used to smoke.
- be able to — ability, usable in all tenses: I was able to finish.
Questions and Negatives
Auxiliaries are the engine of English questions and negatives. To make a question, we put the auxiliary before the subject. To make a negative, we add not after the auxiliary. With present and past simple, where there is no other auxiliary, we use do / does / did.
| Statement | Question | Negative |
|---|---|---|
| She is reading. | Is she reading? | She is not reading. |
| They have arrived. | Have they arrived? | They have not arrived. |
| He works hard. | Does he work hard? | He does not work hard. |
| You can drive. | Can you drive? | You cannot drive. |
What You Will Practise
- Choosing the right primary auxiliary (be, have, do) for each tense
- Forming questions and negatives with auxiliaries
- Matching modal verbs to their meanings
- Using semi-modals such as have to and be able to
Practice Exercises
Strengthen your control of auxiliary and modal verbs with these free exercises.
Grammar Quiz
Multiple-choice questions on auxiliaries and modal meaning.
Cloze Dropdown
Select the correct auxiliary or modal to complete each sentence.
Complete the Sentence
Add the missing auxiliary to form a question or negative.
True or False
Decide whether each auxiliary or modal is used correctly.
Practice What You’ve Learned
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Frequently Asked Questions
What is an auxiliary verb?
An auxiliary verb, also called a helping verb, works with a main verb to build tenses, form questions and negatives, or add meaning. English has primary auxiliaries (be, have, do) that help form tenses, and modal auxiliaries (can, could, will, would, shall, should, may, might, must) that express ideas such as ability, possibility, and obligation. The auxiliary carries grammatical information while the main verb carries the core meaning.
What are the three primary auxiliary verbs?
The three primary auxiliaries are be, have, and do. “Be” helps form continuous tenses and the passive (She is working. The room was cleaned.). “Have” helps form perfect tenses (They have left. I had finished.). “Do” helps form questions, negatives, and emphasis in the present and past simple, where no other auxiliary is present (Do you know? I don't agree. I do care.). These same three verbs can also be used as ordinary main verbs.
How do I use auxiliary verbs to make questions and negatives?
To form a question, put the auxiliary before the subject: She is reading becomes Is she reading? To make a negative, add “not” after the auxiliary: She is not reading. When there is no other auxiliary, as in the present or past simple, use do, does, or did: He works hard becomes Does he work hard? and He does not work hard. The main verb then returns to its base form.
What is the difference between a modal verb and a primary auxiliary?
Primary auxiliaries (be, have, do) help form tenses and change their form for person and tense (is, are, was, has, had, does, did). Modal auxiliaries (can, must, should) never change form, are always followed by a bare infinitive, and add meanings such as ability, permission, possibility, and obligation rather than building tenses. You cannot use two modals together, but you can combine a modal with a primary auxiliary, as in “She must have left.”
What do the modal verbs can, could, may, and might express?
“Can” expresses present ability or possibility and informal permission (I can swim. Can I go?). “Could” is the past of can and also a polite or less certain form (Could you help? It could be true.). “May” expresses formal permission and possibility (May I come in? It may rain.). “Might” expresses a smaller possibility than may (It might snow later.). The differences are often about formality and degree of certainty.
What is a semi-modal verb?
A semi-modal is an expression that has a modal-like meaning but follows ordinary verb grammar. Common examples are have to, need to, ought to, used to, and be able to. Unlike true modals, many of them change form and use “do” for questions and negatives: Do you have to work? They are useful because they fill gaps that true modals cannot, such as expressing obligation in the past (I had to leave) or ability in the future (I will be able to drive).
What is the difference between must and have to?
Both express strong obligation, but the source often differs. “Must” usually expresses obligation that the speaker feels personally or imposes (You must finish this report.), while “have to” usually expresses obligation from outside rules or circumstances (I have to wear a helmet at work.). In the past, must has no past form for obligation, so we use “had to” (Yesterday I had to leave early.). The negatives differ sharply too: “must not” means it is prohibited, while “don't have to” means it is not necessary.
Can I use two modal verbs together?
No, you cannot place two modals side by side. You cannot say “will can”. Instead, replace one modal with a semi-modal equivalent: instead of “will can” use “will be able to” (I will be able to help tomorrow.), and instead of “must can” use “must be able to”. You can, however, combine a modal with the perfect auxiliary have, as in “She might have forgotten” or “You should have called.”
Why is “do” used in some sentences but not others?
We use “do”, “does”, or “did” as an auxiliary only in the present and past simple, and only when there is no other auxiliary or modal in the sentence. If the sentence already has be, have, or a modal, that verb does the work of forming the question or negative. So we say “Is she happy?” and “Can you swim?” without do, but “Does she work here?” with do, because “work” has no auxiliary of its own.
How do modal verbs of deduction work?
Modals can express how certain we are about something. “Must” shows we are almost certain it is true (She must be at home; her car is here.). “Might”, “may”, and “could” show a real possibility (He might be asleep.). “Can't” shows we are almost certain it is not true (That can't be right.). For deductions about the past, we add “have” plus a past participle: She must have left, He might have forgotten, That can't have been easy.