What is a contraction?
A contraction is a shortened form of a word (or group of words) that omits certain letters or sounds. In most contractions, an apostrophe represents the missing letters. The most common contractions are made up of verbs, auxiliaries, or modals attached to other words: He would=He’d. I have=I’ve. They are=They’re. You cannot=You can’t.
Contractions are common in speech—so common that we don’t always take the time to pronounce them precisely, which leads to a particular contraction mistake writers might make if they’re not paying attention.