The short form of was not. You use it everywhere in speech and in friendly writing, with I, he, she, it and any singular thing: I wasn't ready, it wasn't my fault, she wasn't there. For you, we and they the matching word is weren't, because those take were. Two useful facts. First, wasn't is not lazy or wrong — it is what normal spoken English sounds like, and saying "was not" out loud makes you sound either very formal or very cross. English speakers say the full form on purpose when they want to press hard: "I was NOT lying." So the choice carries feeling, not just style. Second, in careful writing — an exam essay, a report, a business letter — the full was not is the safer choice, because contractions look casual on paper. Wasn't also shows up in question tags, where it is extremely common and where learners in Taiwan most often meet it: "He was late, wasn't he?" and the reverse, "He wasn't late, was he?"