句法复杂性 Syntactic Complexity
Theories of syntax assign words to part-of-speech categories (e.g., nouns, verbs, adjectives, conjunctions), group words into phrases or constituents (noun-phrases, verb-phrases, prepositional-phrases, clauses), and construct syntactic tree structures for sentences. For example, some sentences are short and have a simple syntax that follow an actor-action-object syntactic pattern, have few if any embedded clauses, and have an active rather than passive voice. Some sentences have complex, embedded syntax that potentially places heavier demands on working memory. The syntax in text tends to be easier to process when there are shorter sentences, few words before the main verb of the main clause, and few words per noun-phase.
句法理论将词归入语篇类别（如名词、动词、形容词、连词），将集体名词归入短语或成分（名词短语、动词短语、介词短语、从句），并构建句子的句法树结构。例如，有些句子很短，句法简单，遵循 "行为者--动作--对象 "的句法模式，即使有内嵌分句也很少，而且使用的是主动语态而不是被动语态。有些句子的句法复杂，有嵌入式句法，可能会对工作记忆提出更高的要求。如果句子较短，主句的主要动词前只有很少的单词，每个名词相位只有很少的单词，那么文本中的句法往往更容易处理。