Auxiliary verbs are helping verbs that work with a main verb to build meaning. They can show time, aspect, possibility, obligation, permission, and emphasis. In English, auxiliaries also help form questions and negatives without changing the main verb. The main auxiliary groups are be, have, do, and the modal auxiliaries.

Be is an auxiliary when it combines with another verb form. It is used to form continuous tenses with the -ing form, and passive voice with the past participle. Be changes form to match the subject and time, while the main verb stays in the needed form.

Rule	Example
Be + -ing forms the continuous aspect	She is studying now.
Be + past participle forms the passive voice	The window was broken.
The form of be agrees with the subject and tense	They are working today.

Have is an auxiliary when it forms perfect tenses with a past participle. Perfect aspect connects a time to a result or experience, often showing something completed before another time. Have changes form for the subject and tense, while the main verb uses the past participle.

Rule	Example
Have + past participle forms the perfect aspect	I have finished my work.
Had marks the past perfect for an earlier past time	She had left before noon.
Perfect aspect can express experience up to now	They have visited Paris.

Do is an auxiliary used to form negatives and questions in simple present and simple past when there is no other auxiliary. It also appears in short answers and can add emphasis in affirmative statements. When do is auxiliary, the main verb stays in the base form.

Rule	Example
Use do to make negatives in simple present or past	He does not like coffee.
Use do to make questions in simple present or past	Do you work here?
Use do for emphasis in affirmative statements	I do want to help.
After do, the main verb stays in the base form	She did not go yesterday.

Modal auxiliaries add meanings like ability, possibility, permission, advice, and obligation. Common modals include can, could, may, might, must, shall, should, will, would, and ought to. Modals do not take -s, do not use do-support, and are followed by the base form of the main verb.

Rule	Example
Modals are followed by the base verb	She can swim.
Modals do not change for third person singular	He can drive.
Negatives and questions use the modal itself, not do	Can you help?
Modals express meanings beyond time, such as obligation or possibility	You must stop.

When multiple auxiliaries appear together, English follows a common sequence. Modals come first, then perfect have, then continuous or passive be, then the main verb. Each auxiliary determines the form of the next verb, so correct order helps keep meaning clear.

Rule	Example
Modal + have + past participle	She might have missed the call.
Modal + be + -ing	They will be waiting.
Modal + have + been + -ing	I would have been working.
Modal + have + been + past participle for passive perfect	The files may have been deleted.

Auxiliaries help form questions by moving to the beginning of the clause. If there is an auxiliary already, it inverts with the subject. If there is no auxiliary, do is added, and the main verb stays in the base form.

Rule	Example
If an auxiliary exists, invert auxiliary and subject	Are you ready?
With have or modals, invert the auxiliary	Have they arrived?
If no auxiliary exists, add do and use base verb	Did she call you?

Auxiliaries create negatives by adding not after the auxiliary. This includes be, have, do, and modals. If there is no auxiliary in a simple present or past statement, do is added and the main verb stays in the base form.

Rule	Example
Place not after the auxiliary	She is not coming.
Modals take not directly	You should not worry.
Add do if there is no auxiliary	We do not know.

Auxiliaries often contract in everyday English, especially with pronouns. Not also contracts with many auxiliaries. Contractions are common in speech and informal writing, while full forms are more formal or used for emphasis and clarity.

Word/Phrase	Definition	Example
I am → I’m	Be contracted with the subject	I’m late.
She is → She’s	Be contracted with the subject	She’s working.
They have → They’ve	Have contracted with the subject	They’ve finished.
do not → don’t	Negative contraction with do	I don’t agree.
will not → won’t	Irregular negative contraction with will	He won’t come.

Many auxiliary verbs can also act as main verbs. As auxiliaries, they support another verb and often help grammar like tense, questions, or negatives. As main verbs, they carry the core meaning and can appear alone, often with a different structure.

Rule	Example
Be can be a main verb for identity or state	She is a teacher.
Have can be a main verb for possession or experience	They have a car.
Do can be a main verb for actions or tasks	I do my homework.
As auxiliaries, these verbs combine with another verb form	They have eaten already.

Auxiliary verbs are be, have, do, and the modals, and they work with a main verb to build meaning and grammar. They form continuous and passive structures with be, perfect structures with have, and questions and negatives with do when needed. Modals add meanings like ability and obligation and are followed by the base verb. Recognizing auxiliaries helps you form correct questions, negatives, and multi-verb verb phrases.