How do you know when to use "a" or "the" is a sentence? For example, "Are you riding a bus?" or "Are you riding the bus?"

A, An, The are articles.

Use "a" or "an" with a singular count noun when you mean "one of many," "any," "in general." Ex: John is a boy. Use "the" with any noun when the meaning is specific; for example, when the noun names the only one (or one) of a kind. Ex: I live on the Earth.