To become a member of the civic association, one need only attend three meetings, and to pay his fees regularly. (TOEFL)
书中答案规定将 to pay 改为 pay, 这显然是将 need 看成了情态动词 (need attend and pay)。但是许多语法书都强调指出, 作为情态动词使用的 need 只能用于否定句和疑问句, 而不能用于肯定句。例如:
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She needn't stay if she doesn't want.
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Does she need stay any longer?
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He needn't work so late, need he? (Hornby)
这样看,考题和答案似乎有问题。
作为情态动词使用的 need, 除用于否定句和疑问句之外, 也可用于肯定句中。但是, need 用于肯定句是有条件的。这个条件是: 肯定句中必须含有某些暗含某种否定含义的词语, 例如 only, but, all, before 等等, 或者说句子暗含着某种否定含义。
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There was another hour before I need go out into the cold. (Snow) 这句话的否定含义是: I don't need to go out into the cold now.
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One need only consider the facts. (MED) 这句话的否定含义是: 只需考虑这些事实, 而勿需去做别的事情。上面的托福试题便属于这种情况。
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He need do it but once. (Leech) 这句话的否定含义是:他不必做两次以上。
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All he need do is state his opinion clearly. (Jacobsson) 这句话的否定含义是:除了清楚地陈述自己的观点之外,他不需再做任何事情了。
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All you need do is to take a taxi from the airport. (LEG, § 11·49) 你需要做的只是从机场乘一辆出租汽车。
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Americans, after all, come forward to embrace the foreign TV-man so welcomingly that all he need do is stand there and occasionally put out his foot. (The Observer) 要知道,美国人上前来如此欢迎地拥抱这位外来的电视人,以至他需要做的一切,就是站在那儿,偶尔伸伸脚。(RWRT)
据 RWRT 的作者们解释说, that all he need do is stand there... 可以释义为 that he need do nothing more than stand there...
下面的句子跟上面的考题很类似：
One need think only of South Africa in 1972, New Zealand the following year and Argentina in 1981. (The Times) 我们只需要想一想 1972 年的南非, 1973 年的新西兰和 1981 年的阿根廷。
因此,上面托福考题中的 One need only attend... and pay 是正确的。
然而，RWRT的作者们同时指出，如果让美国人或加拿大人来写上面的句子，也许他们更喜欢选用all he needs to do和one needs only to think。
还应指出，情态助动词还可用于比较结构的肯定句中：
Standards are lower than they need be. (Jacobsson) 标准比所需要的要低。