A determiner is a word that comes before a noun to help specify what you're talking about. Think of determiners as signposts that point to nouns and tell you important information about them.

The most common determiners are the, a, and an. When you say “the dog,” you're pointing to a specific dog that both you and your listener know about. When you say “a dog,” you're talking about any dog, not one in particular. Other determiners include this, that, these, those, my, your, some, many, each, and every.

Determiners answer questions like: Which one? How many? Whose is it? When you write “this book is interesting,” the word this is a determiner showing exactly which book you mean. When you say “my sandwich” or “three cookies” or “several problems,” those first words are all determiners giving crucial information about the nouns that follow.

In English, most singular countable nouns need a determiner in front of them. You wouldn't usually say “Dog is hungry,” you'd say “The dog is hungry” or “My dog is hungry.” Understanding determiners helps you recognize how sentences are built and why certain words appear where they do.