前缀（prefix），顾名思义，加在英语单词（词根root）前的一个部分。通常情况下，前缀可以改变单词的意思或词性。

前缀un- 构成的常见词：

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