An auxiliary verb, also known as a helping verb, is a verb that accompanies a main verb to form a verb phrase. It helps to express tense, mood, voice, or aspect. Common auxiliary verbs include be, have, and do. For example, in the sentence "She is running," the word "is" is the auxiliary verb that helps indicate the present continuous tense. Auxiliary verbs can also be used to form questions and negatives. In the sentence "Do you like ice cream?" the word "do" serves as an auxiliary verb to create a question. Similarly, in "He does not play soccer," "does" helps to form a negative statement.