在英语语法中，封闭类指的是虚词的类别，即词性（或词类）——不容易接受新成员。英语中的封闭类包括代词、限定词、连词和介词。相比之下，开放类词汇包括名词、词汇动词、形容词和副词。

实例和意见

"[C]losed-class words are those belonging to the grammatical, or function, classes...Function words in English include conjunctions (and, or), articles (the, a), demonstratives (this, that), and prepositions (to, from, at, with). To take one specific case, consider the word and. The essential feature of the word and is that it functions grammatically to conjoin words and phrases, as seen in the combination of noun phrases the woman and the man. Any change in membership in such a class happens only very slowly (over centuries) and in small increments. Thus, a speaker of English may well encounter dozens of new nouns and verbs during the coming year; but it is extremely unlikely that the English language will acquire a new article (or lose a current one) in the coming year (or even in the speaker's lifetime)." —Adrian Akmajian, et al., Linguistics: An Introduction to Language and Communication. MIT, 2001 "Prepositions have gradually expanded their membership somewhat by admitting participles such as including, concerning, but the remaining classes are very resistant to the introduction of new items. This has been noticeable in recent years when attempts have been made to find gender-neutral pronouns." —Angela Downing and Philip Locke, English Grammar: A University Course, 2nd ed. Routledge, 2002

封闭类词的功能

"Closed-class words or 'function words' are limited in number and act as markers or guides to the structure of a sentence. The role of articles is to signal nouns. Prepositions mark special relationships between persons, objects, and locations. Conjunctions are connectors that link actors or objects, and specify relationships between clauses in the sentence. Open- and closed-class words occupy certain slots in sentences and set up a frame for interpreting the interrelationships between actors, actions, and objects." —Diane McGuinness, Language Development and Learning to Read. MIT, 2005

开放类词语演变为封闭类词语

"The closed classes include pronouns (you, them), modal verbs (could, must), determiners (a, the), prepositions (of, in), and conjunctions (and, but). New members of these classes are not added to the language very often. Instead, they tend to gradually evolve from lexical words in a process called grammaticalization. For example, the lexical verb go means 'to move (toward a goal).' But its progressive form be going (to) has evolved into a grammaticalized prospective (future) marker, as in She's going to love her gift. The 'movement' meaning of go has been bleached out of the grammaticalized version, and so the going in be going to can be considered to be a function word, rather than a content word. The closed classes represent a more restricted range of meanings, and the meanings of closed-class words tend to be less detailed and less referential than open-class words." —M. Lynne Murphy, Lexical Meaning. Cambridge University Press, 2010