Auxiliary verbs are verbs that accompany a main verb (also called a principal verb) to form other tenses. Auxiliary verbs are used to form many different tenses and are also necessary to make negative statements and questions. Because of the nature of their function, auxiliary verbs are often referred to as “helping verbs.”

Here is the complete list of auxiliary verbs:

be	do	must	should
can	have	need	used to
could	may	ought to	will
dare	might	shall	would

Let’s look at some auxiliary verbs in action to see how they work and how they’re different from the main verb.

Dad will arrive at the airport by 7:30 tomorrow morning.

The complete verb phrase in this sentence is will arrive. The main verb is arrive, which describes the primary action taking place in the sentence. The auxiliary verb is will. Adding will puts the action of arriving in future tense, letting us know that Dad will be arriving at some point in the future. Either will or arrive by itself would not be correct or sensible in this case. To properly convey the intended meaning of the sentence, arrive needs the help of will.

You ought to eat healthy.

The complete verb phrase here is ought to eat, the main verb is eat, and the auxiliary verb is ought to. Notice that ought is nearly inseparable from to, and the pair of them function together as one auxiliary verb to mean the same thing as should. Keep the two auxiliary verbs that do this in mind (the other one being used to), especially to avoid confusion when dealing with prepositions or infinitives. Also note that ought can be used without to, but such instances are rare and often seen in archaic writing.

Is Thomas going to his daughter’s violin recital?

This time the main verb is going, and it is accompanied by the auxiliary verb is to form the present continuous tense. Although is is perfectly capable of functioning on its own, in this sentence, like the first example, neither is nor going would be correct or sensible on its own. Also notice that the auxiliary verb is necessary to make this sentence a question. Without the use of is in its proper place, the sentence would be a declarative statement. The correct intended meaning of the sentence is dependent upon not only the cooperation of the main verb and its auxiliary, but also the proper placement of each.

Now that we understand the basic function of auxiliary verbs, let’s look at them in a bit more detail. The list of auxiliary verbs can be further broken down into three smaller sections: primary auxiliary verbs, modal auxiliary verbs, and semi-modal auxiliary verbs.

Primary Auxiliary Verbs

The primary auxiliary verbs are be, do, and have. The primary auxiliary verbs are the most common of the bunch and are unique in that they can stand alone as main verbs and conjugate accordingly. Let’s look at some examples.

Alice is trying her best.

Is (a form of be) is functioning as an auxiliary verb to trying to form the present continuous tense.

Alice is over there.

Now is is standing alone as the only verb in the sentence and makes perfect sense without being paired with another verb.

Trent has been planning a trip to Iceland since last month.

In this example, has and been are both auxiliary verbs that form the present perfect continuous tense with the main verb planning. Again, none of the three verbs would be correct or make sense without the others in this context.

Adrian has an impressive collection of soccer memorabilia.

This time has stands alone as a stative verb to indicate possession and needs no auxiliary verb to complete its meaning.

The board of directors will have a meeting on Thursday.

This example again shows have as the main verb being helped by the auxiliary verb will to form the future tense. When a primary auxiliary verb is not functioning as an auxiliary, it can be accompanied by other auxiliary verbs, just like any other main verb can.

Modal Auxiliary Verbs

Modal auxiliary verbs are can, could, may, might, must, shall, should, will, and would. As you might have already guessed, modal auxiliary verbs are unique in that they cannot stand alone as a main verb and cannot be conjugated into any other forms. These verbs only serve as auxiliaries. They must accompany another verb, and that verb must be in its base form (its unconjugated form as it is written in the dictionary). Modal verbs may also be referred to as mode verbs or modals. Notice that with the exception of will, modal auxiliary verbs are not even used for the purpose of forming tenses, but rather for indicating ability, intent, permission, suggestion, or obligation—the mode in which the verb takes place. Hence the nomenclature.

I must finish this essay before Friday.

In this sentence, must functions as an auxiliary verb for the main verb finish to indicate a duty or obligation. Notice that although must does not change the tense, it is still necessary to convey the intended meaning. It changes the mode—the manner in which the verb takes place.

John might just stay home this weekend.

The auxiliary verb might works with the main verb stay here. As with must, might does not change the tense; it changes the mode to, in this case, indicate possibility.

Semi-modal Auxiliary Verbs

Dare, need, ought to, and used to are called semi-modal verbs because they don’t fit neatly into either of the latter categories.

Dare and need are tricky because they function as main verbs or modal auxiliary verbs, although their use as modal auxiliary verbs is becoming increasingly uncommon. When they function as modal auxiliary verbs, they do not conjugate and must be followed by a main verb in its base form. Let’s look at some examples of each way dare and need can be used.

Angus will need to renew his driver’s license eventually.

Need functions as the main verb and is accompanied by the auxiliary verb will to form the future tense.

You need not worry about the quiz tomorrow.

In this case, need is functioning as a modal auxiliary verb accompanying the main verb worry. This kind of construction is rare in modern usage. We would usually say something more like, “You do not need to worry about the quiz tomorrow,” in which case need would be the main verb assisted by the primary auxiliary verb do. It is also worth noting that when need is used as a modal auxiliary verb, it will almost exclusively be used in negative sentences.

Steve was dared to talk to Angela.

Here dared functions as a main verb helped by was and even has an object—the infinitive to talk.

Jack did not dare to question Mr. Perez.

In this construction, dare is again a main verb, but this time it is intransitive, having no object. Notice that in constructions like this one, the infinitive following dare functions as an adverb modifying dare. The to with such infinitives may also be left out, as the meaning is still understood or assumed without it.

I dare not skip class tomorrow.

Here dare is a semi-modal auxiliary verb assisting the main verb skip. Like need, it is almost always in a negative statement when used in such constructions and is rarely used this way.

How dare you disrespect a teacher?

Everyone is very familiar with this use of dare as a semi-modal auxiliary verb. Here dare functions as an auxiliary accompanying the main verb disrespect. This kind of construction is the most common way to use dare as a semi-modal auxiliary verb.

Ought to and used to are similar in that they both end with to, thereby making the main verb that follows them in their auxiliary usage an infinitive. For this reason, you may see ought to and used to as subjects of debate, especially considering their similarities with other common constructions like need to and have to, in which have and need are conjugatable main verbs modified by infinitives. However, with the occasional exception of used to, ought to and used to do not conjugate as auxiliaries and fit in the semi-modal category much better than they would anywhere else.

Ought to is always used in an auxiliary capacity, never as a main verb. It does not conjugate, and, as with dare, the to that goes with it may often be omitted from its accompanying infinitive.

We ought to go home early.

Ought to accompanies the main verb go to show preference or obligation, having the same meaning as the modal auxiliary verb should.

“Whose mouths must be stopped, who subvert whole houses, teaching things which they ought not, for filthy lucre’s sake.” –Titus 1:11

In older usage, such as that used in the King James Bible, the entire infinitive phrase may be dropped and understood from the context. In this example from Titus 1:11, the understood meaning is that the people described are teaching things which they ought not to teach, but the context makes the meaning abundantly obvious with or without the presence of the infinitive, making it perfectly acceptable and understandable to leave it out entirely.

Used to is quite quirky. Be used to, get used to, and become used to are main verbs and can be conjugated and have objects of their own.

Earl is still getting used to working night shifts.

Notice that in main verbs, used to does not change, but the preceding verb (be, become, or get) is the one that conjugates. In this example, the object of getting used to is working, which is a gerund. Be careful not to confuse a following gerund as part of the verb phrase.

As a semi-modal auxiliary verb used to precedes the base form of another verb.

Tanner used to play football.

Here used to accompanies the main verb play.

Richie did not use to eat his vegetables.

Used to is particularly interesting when forming questions and negative statements. In such cases, the necessary use of the auxiliary verb did creates the past tense, meaning that used to technically becomes use to, although it is not uncommon to see it written as used to anyway.