Is is the most common form of the verb “to be,” which we use constantly when describing what something is like, where it is, or what it is doing. When you say “The sky is blue” or “My backpack is in my locker,” you're using is to connect the subject (the sky, my backpack) to information about it.

Is works for singular subjects in the present tense: “She is happy,” “The book is interesting,” “Today is Tuesday.” For plural subjects or the pronoun “you,” we use are instead: “They are happy,” “The books are interesting.” For “I,” we use am: “I am ready.”

This tiny word appears in nearly every conversation and piece of writing, acting like glue that holds sentences together. Without is and its relatives, we'd struggle to describe the world: we couldn't say what things are, where they are, or how they are.