前綴（prefix），顧名思義，加在英語單詞（詞根root）前的一個部分。通常情況下，前綴可以改變單詞的意思或詞性。

前綴un- 構(gòu)成的常見詞：

able - unable

clear - unclear

happy - unhappy

like - unlike

cover - uncover

necessary - unnecessary

important - unimportant

crowded - uncrowded

comfortable - uncomfortable

lucky - unlucky

certain - uncertain

sure - unsure

kind - unkind

known - unknown

willing - unwilling

usual - unusual

pleasant - unpleasant

paid - unpaid

likely - unlikely

doubted - undoubted

expected - unexpected