Many Chinese learners of English know just enough grammar to go wrong. "It seems I have seen such an expression several times", they say to themselves, "but surely it is not grammatical. I must alert it so as to make it grammatical." Their narrowly grammatical conscience, however, often causes them to change idiomatic English into unidiomatic English. For example, they may find "The boy acts contrary to his parents' wishes" ungrammatical, and to make the sentence grammatical change the "contrary" into "contrarily" – with the result that the sentence would not read like an English sentence to an Englishman. It is true that from a narrowly grammatical point of view, the adjective contrary here should be replaced with the adverb "contrarily". But idiom has decided that the adjective is the word required here, and idiom there is no disputing. (Perhaps it would be better to say that here idiom insists on the use of the adverb "contrary" instead of the adverb "contrarily".)
There are several forms of expression that are purely idiomatic and do not admit of grammatical analysis. The word "busy", for example, may be used immediately before a gerund looking like a participle, as in "He is busy writing letters".
In your reading, reader, take care to note down those idiomatic constructions that you do not find grammatical enough. Commit them to memory and try to use them in your writing. Above all, remember that you may happen to be one of those who, as I said in the above, know just enough grammar to go wrong.
You must not think, however, that idiom and grammar are always incompatible. What is idiomatic is far more often grammatical than ungrammatical. I mean simply that not every idiomatic construction is grammatically explainable, nor is every strictly grammatical construction idiomatic.
NOTES
1. Go Wrong 错误
2. It seems=It seems that
3. say to themselves 想
4. narrowly grammatical conscience 对于语法的狭隘的观念
5. with the result that... 结果是...；以至于...
6. idiom there is no disputing=there is no disputing idiom 不能反对习语
7. admit of 可能；容许
8. note down 记下
9. Above all 尤要者
