参考答案： An English idiom has the established form, one that has been accepted by traditional usage. The words in idiomatic expressions cannot, as a rule, be deleted, added to, replaced by synonymous words, or put in a different order, without affecting or destroying the figurative meaning. The following examples may help illustrate the structural invariability of English idioms. The structural invariability can be observed by the following two examples: See the structural invariability with a comparison between the following two sentences: *I expected him to kick the bucket and kick it he did. I asked him to write a letter to my aunt, but not to write it in English. To sum up, the structural invariability of an English idiom is at least two-fold in sense. In the first place, it implies that English idioms cannot be freely made up at the speaker’s will but are reproduced in speech as ready-made units; secondly, unlike free phrases which may vary according to the needs of communication member, constituent...