惯用语是个固定的词组，有一定的`语义，是不可分割的统一体，其整体的意思往往不能从组成惯用语的各个词的原有意思中猜测而得。此外，惯用语的各个组成部分不可任意拆开或替换。但是某些惯用语，在特殊情况下，容许做些更动，从而增添活用性。
现在从下面的四点举例说明：
⒈惯用语组成部分的替换，例如：
● pull a long face（愁眉苦脸）pull an unhappy face, 如：
The football coach pulled an unhappy face when his team was defeated.
● a stitch in time saves nine（及时补救）a word in time saves nine, 如：
The two were going to quarrel, but their friend cut in to appease them. A word in time saves nine.
⒉惯用语组成部分分开，例如： ● get an upper hand（占上风）In a debate competition, a quick-witted debater can get an upper cut, if not the upper hand. ● cannot see the wood for the trees（见树不见林）He sees too many trees, but not the whole wood.
⒊惯用语组成部分的扩充，例如： ● put into practice （实行）put into widespread practice, 如：
Davids proposal was well accepted, but no real effort was made to put it into widespread practice.
● keep company （结交朋友）keep disreputable company, 如：
