Harper Dictionary of Contemporary Usage 说 the necessity 只能用 of 短语或 for 短语来作定语，而绝不能用动词不定式来作定语。而 need 作名词时，却可以用动词不定式作定语。是这样吗？
名词 necessity 主要是用 of 短语或 for 短语来作后置修饰语。例如：
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What's the necessity of leaving so early? (FWF) 有必要离开这么早吗?
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... the necessity of keeping water uncontaminated(EED) 保持水不受污染的必要性
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There is no necessity for alarming him at this time. (HDCU) 现在没有必要去惊动他。但是，在较少的情况下，也有人使用动词不定式来作名词 necessity 的后置定语：
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There is no necessity to hurry. (Wood) 没有必要着忙。
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I don't think there is any necessity to bring him into the affair yet. (Du Maurier) 我认为现在没必要让他卷入这个事件。
名词 need 虽可用 of 短语或 for 短语来作定语,但更多的是动词不定式来作定语:
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There is no need for alarm. (Wood) 没必要吃惊。
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We have no need of your advice. (Wood) 我们不需要你的建议(忠告)。
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There is no need to worry at all. (Du Maurier) 根本没有必要担心。
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She felt no need to speak. (EED) 她觉得没有说话的必要。
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There is no need to alarm him at this time. 没必要在这时候惊动他。