A suffix is a letter or group of letters added to the end of a word to change its meaning or how it works in a sentence. When you add the suffix -ness to the word “happy,” you get “happiness.” When you add -er to “teach,” you get “teacher.” The suffix transforms the original word into something new.

Suffixes follow reliable patterns. Adding -ly often turns an adjective into an adverb: “quick” becomes “quickly.” Adding -ful can turn a noun into an adjective: “hope” becomes “hopeful.” Some suffixes change a word's tense: -ed makes “walk” into “walked,” showing it happened in the past.

Understanding suffixes helps you decode unfamiliar words. If you see “careless,” you can break it into “care” plus the suffix -less, which means “without.” Suddenly a new word makes sense: “without care.” If you encounter “mountainous,” you recognize “mountain” plus -ous, which often means “full of” or “having the quality of.”

The opposite of a suffix is a prefix, which attaches to the beginning of a word instead. Between prefixes and suffixes, you can build and understand thousands of words from a much smaller set of roots.