The sound of the falls could still be heard. The sound of the falls still reached our ears.

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The reason that he left college was that his health became impaired. Failing health compelled him to leave college.

It was not long before he was very sorry that he had said what he had. He soon repented his words.

As a rule, avoid making one passive depend directly upon another. (来源：EnglishCN英语博客基地)

Gold was not allowed to be exported.   It was forbidden to export gold (The export of gold was prohibited). (来源：英语麦当劳www.EnglishCN.com)

He has been proved to have been seen entering the building. It has been proved that he was seen to enter the building. (来源：英语麦当劳－英语快餐EnglishCN.com)

In both the examples above, before correction, the word properly related to the second passive is made the subject of the first. (来源：英语博客 http://space.englishcn.com)

A common fault is to use as the subject of a passive construction a noun which expresses the entire action, leaving to the verb no function beyond that of completing the sentence. (来源：EnglishCN英语问答中心[e问e答])

A survey of this region was made in 1900. This region was surveyed in 1900. (来源：英语麦当劳－英语快餐EnglishCN.com)

Mobilization of the army was rapidly carried out. The army was rapidly mobilized.

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Confirmation of these reports cannot be obtained. These reports cannot be confirmed.

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Compare the sentence, "The export of gold was prohibited," in which the predicate "was prohibited" expresses something not implied in "export." (来源：英语论坛 http://bbs.englishcn.com)

4. Put statements in positive form. (来源：EnglishCN英语博客基地)

Make definite assertions. Avoid tame, colorless, hesitating, non-committal language. Use the word not as a means of denial or in antithesis, never as a means of evasion. (来源：英语麦当劳－英语杂志 www.EnglishCN.com)

He was not very often on time. He usually came late.

He did not think that studying Latin was much use. He thought the study of Latin useless. (来源：英语麦当劳－英语学习门户 EnglishCN.com)

The Taming of the Shrew is rather weak in spots. Shakespeare does not portray Katharine as a very admirable character, nor does Bianca remain long in memory as an important character in Shakespeare's works. The women in The Taming of the Shrew are unattractive. Katharine is disagreeable, Bianca insignificant. (来源：英语电影下载 http://bt.englishcn.com)

The last example, before correction, is indefinite as well as negative. The corrected version, consequently, is simply a guess at the writer's intention.

All three examples show the weakness inherent in the word not. Consciously or unconsciously, the reader is dissatisfied with being told only what is not; he wishes to be told what is. Hence, as a rule, it is better to express a negative in positive form.

not honest dishonest (来源：英语e问e答 http://ask.englishcn.com)

not important trifling

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did not remember forgot (来源：英语图片 http://photo.englishcn.com)

did not pay any attention to ignored

did not have much confidence in distrusted (来源：英语麦当劳－英语杂志 EnglishCN.com)

The antithesis of negative and positive is strong: (来源：英语美食指南 http://food.englishcn.com)

Not charity, but simple justice.

Not that I loved Caesar less, but Rome the more. (来源：英语聊天室 http://chat.EnglishCN.com)

Negative words other than not are usually strong: (来源：英语麦当劳 http://www.EnglishCN.com)

The sun never sets upon the British flag.