The prefix un- attaches to the beginning of words to reverse their meaning or indicate the opposite. When you're unhappy, you're not happy. When you unlock a door, you reverse the action of locking it. When something is unfinished, it's not finished.

This tiny syllable is one of the most productive tools in English. You can stick it onto thousands of words to flip their meaning: unfair, unkind, uncomfortable, unbelievable. Sometimes un- undoes an action: you unpack your suitcase, untie your shoes, or unwrap a present.

Not every word accepts un- as a prefix. You can say unhappy but not unsad. You can unlock something but you wouldn't unopen it. English speakers have collectively decided which combinations sound right, and those are the ones that stick. When you encounter a new word starting with un-, you can usually figure out its meaning by identifying the root word and reversing it. If predictable means you can predict it, then unpredictable means you cannot predict it.