An auxiliary verb is a type of verb used to modify or accompany another verb, called the main verb, to change the meaning or intention of that verb. These are often verbs that can be main verbs in some usages, but act as an auxiliary in other uses. This type of auxiliary verb is often used to change the tense of the main verb. Some auxiliary verbs are never used as main verbs, such as modal verbs that can be used to indicate or request permission, ability, or probability of an action.
Also called a helping verb, an auxiliary verb or aux verb is used with a main verb to modify or alter the specific meaning of the main verb. One of the most common uses of auxiliary verbs is to change the tense of the main verb in a particular sentence. This is usually done with either the verb “is,” which is often associated with the concept of “to be” or the verb “have.”
“Is” or “to be” is a concept found in many different languages and is one of the most important and common verbs in the English language. While it can be used as a main verb, such as in the sentence “the cat is furry,” it can also be an auxiliary verb. When used as an auxiliary verb, “is” typically creates either passive voice or the progressive aspect of a present tense statement. Passive voice is created when something is being acted upon, rather than performing an action, such as “The ball was thrown by the man,” which is passive when compared to “The man threw the ball.”
While passive voice is typically considered inferior to active voice in most writing, “is” can also be used to create present progressive tense for a main verb. In the sentence “The woman runs every day” the main verb merely indicates that she runs without any indication of when it might occur. If the sentence used “is” as an auxiliary verb to change the sentence to “The woman is running,” then it indicates that the action is currently happening due to progressive voice. Past perfect tense is created in much the same way, but uses “have” instead of “is” such as “The dog has eaten a steak every day, which makes him quite happy” to indicate an ongoing action occurred in the past but produced a current state of being.
An auxiliary verb is also commonly used to indicate modality, often called a modal verb. In this usage, the auxiliary verb modifies a main verb by indicating permission, ability, or probability. In English, for example, the modal verb “may” usually indicates permission in a statement like “you may eat the last piece of cake.” Other common modal verbs include “can,” to indicate ability in a sentence like “I can run very fast,” and “should,” “must,” and “might” to indicate probability in a statement like “I must pay this bill and I should buy some new shoes.”