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Be, do, and have function as primary auxiliary verbs, which are helping words that combine with main verbs to indicate aspect, voice, questions, and negation. An auxiliary verb changes its form for subject agreement and tense, leaving the main verb to specify the concrete action. These three words also serve as independent main verbs, but when they help another verb, they follow strict pairing rules with base verbs, -ing forms, and past participles.

Are you repairing that touring bicycle right now?