Wasn't is the contracted form of “was not.” It's one of the shortest and most useful words in English for describing what didn't happen or what something used to not be.

You use wasn't when talking about a single person or thing in the past. “The movie wasn't scary at all.” “She wasn't home when I called.” “It wasn't raining yesterday.” The contraction makes sentences flow more naturally in everyday speech and writing.

The key is knowing when to use wasn't versus weren't. Use wasn't for singular subjects (he, she, it) and weren't for plural subjects (we, they). So you'd say “He wasn't ready,” but “We weren't ready” and “They weren't ready.”

In very formal writing, like academic papers or official documents, you might write out “was not” instead of using the contraction. But in most situations, from friendly emails to creative stories, wasn't sounds more natural and conversational.