“A,” the Indefinite Article

Before we finish, let’s get to the rule for using the indefinite article “a.” It’s called the indefinite article because you use it when you’re talking about something that you’re not trying to distinguish from other things. If you say, “A cat crossed the road,” it could be any cat. If you say, “I wish a cat would cross the road,” there might not even be a cat.

Summary

In short, with countable singular nouns, you have to have a determiner. Use whatever  determiner you need; in particular, use “the” if you’re distinguishing the noun from other things; use “a” if you’re not. With proper nouns, plural nouns, and mass nouns, determiners aren’t necessary, though you can still use them depending on the meaning you’re after; but remember not to use  “a” or any other determiner that implies counting with a mass noun.