Final answer:
In the sentence ‘The girl is riding her bike’, 'is' exists as a helping verb that helps the main verb 'riding' to convey the action happening. The verb 'riding' is the action verb as it depicts the girl's action.
Step-by-step explanation:
In the sentence 'The girl is riding her bike', the verb 'is' serves as a helping verb. A helping verb (also known as an auxiliary verb) helps the main verb of the sentence - in this case, 'riding' - to express various nuances of time, voice, mood, etc. For example, when we say 'The girl is riding her bike', the helping verb 'is' conveys that the action is happening currently, at this very moment. An action verb, on the other hand, expresses the action that is happening. In the provided sentence, 'riding' is the action verb because it's the action the girl is performing.
Learn more about Helping verbs