Prefixes
A prefix is a group of letters placed before the root of the word. For example, the word "unhappy" consists of the prefix "un" which means "not". You combine the prefix with the root word, "happy", and you have the word "unhappy".
The prefixes il, im, in, and ir tell information about relationships between things. All four prefixes mean "not".
The word illegal means "not legal" or "against the law". If you know the definition of a base word, you can figure out the meaning of a new word formed by adding a prefix that means "not".
Prefixes also add meaning to base words. In most cases, the spelling of the base word does not change. Instead, the prefix is simply added to the beginning of the word.