规则19 用相似的形式表达并列的思想[rule-19]

这一原理，即平行结构原理，要求内容与功能相似的表达在外形上也相似。形式的相似使读者更容易识别内容与功能的相似。人们熟悉的《八福》就例证了平行结构的长处。

Blessed are the poor in spirit: for theirs is the kingdom of heaven. Blessed are they that mourn: for they shall be comforted. Blessed are the meek: for they shall inherit the earth. Blessed are they which do hunger and thirst after righteousness: for they shall be filled.

不熟练的写作者常常违反这一原理，错误地相信不断改变表达形式有价值。当重复一个陈述以强调它时，写作者可能需要改变其形式。除此之外，写作者应遵循平行结构原理。

• 误：Formerly, science was taught by the textbook method, while now the laboratory method is employed.
• 正：Formerly, science was taught by the textbook method; now it is taught by the laboratory method.

左边那种写法给人印象是写作者犹豫不决或胆怯，似乎没有能力或不敢选定一种表达形式并坚持下去。右边那种写法表明写作者至少做出了选择并坚持了它。

按照这一原理，适用于一个系列中所有成员的冠词或介词，要么只在第一个词项前使用，要么在每个词项前重复。

• 误：the French, the Italians, Spanish, and Portuguese
• 正：the French, the Italians, the Spanish, and the Portuguese

• 误：in spring, summer, or in winter
• 正：in spring, summer, or winter (in spring, in summer, or in winter)

有些词在某些习惯用法中需要搭配特定的介词。当这类词被组合进一个复合结构时，必须把所有适当的介词都包括进去，除非它们相同。

• 误：His speech was marked by disagreement and scorn for his opponent's position.
• 正：His speech was marked by disagreement with and scorn for his opponent's position.

关联表达（both, and；not, but；not only, but also；either, or；first, second, third；等等）后面应跟相同的语法结构。违反这条规则的许多情况都可以通过重新安排句子来纠正。

• 误：It was both a long ceremony and very tedious.
• 正：The ceremony was both long and tedious.

• 误：A time not for words but action.
• 正：A time not for words but for action.

• 误：Either you must grant his request or incur his ill will.
• 正：You must either grant his request or incur his ill will.

• 误：My objections are, first, the injustice of the measure; second, that it is unconstitutional.
• 正：My objections are, first, that the measure is unjust; second, that it is unconstitutional.

也许会有人问：如果你需要表达相当多——比如二十个——相似的想法，该怎么办？难道你必须写出二十个同型的连续句子吗？仔细考察之后，你很可能会发现这个困难是想象出来的——这二十个想法可以归类分组，你只需在每组之内运用这一原理。否则，最好用表格的形式来陈述，以避开这一困难。