Prefix/ˈpriːfɪks/ Definition 1 of 4Pro Tip 1/3Meaning Alteration Prefixes transform words into antonyms or give them a new meaning. Adding 'dis-' to 'agree' forms 'disagree', indicating opposition. Pro Tip 2/3Common Prefixes Some prefixes are really common and appear in many English words. 'Pre-' is a common prefix in 'preview', meaning 'before'. Pro Tip 3/3No Spelling Change Adding a prefix doesn't typically change the spelling of the root word. 'Un-' + 'known' stays as 'unknown' with no spelling change.