• All English words can be classified as belonging to a “closed class” or an “open class.” (Closed-class words are also called “function words” or “grammar words” and open-class words are also called “lexical words” or “content words.” Closed classes are called “closed” because new words are seldom added to them. By contrast, new items are regularly added to the open classes. The entire noun word class is made up of open-class items as is the entire sub-class of full verbs. All determiners, pronouns, conjunctions, prepositions are closed class. The adverb word class is partly open and partly closed.)
• closed classes and meaning:
The meaning of an open-class item such as a noun or a verb can be given in a dictionary where words are defined in isolation. By contrast, the meaning of closed-class words cannot be explained in isolation because it is tied up with the grammatical structures that they are part of. This is particularly true of determiners and primary verbs. For example, any explanation of the “meaning” of the article, “the” will have to focus on the fact that the word is used to introduce a certain type of noun phrase; and likewise, any explanation of the meaning of the primary verb “do” will have to stress the role it plays in forming questions and negative statements. Correct use of modal auxiliaries requires an understanding of how they operate in complex verb phrases even though the distinctions between them can be seen as a matter of difference in “dictionary” meaning. Conjunctions are similar in that they share the important grammatical function of marking coordination or subordination even though they all have meanings which extend beyond that function. Prepositions signify position, direction etc in a variety of ways that can explained in a dictionary but as a group they are distinguished by their role in indicating sentence structure — introducing prepositional phrases.
• closed-class adverbs
Most English adverbs are formed by adding “ly” to an adjective and are open class. There are many closed-class adverbs, however: This group includes the most “adjuncts” of time and place: here, there, then, now, today, yesterday, and compound adverbs such as nowhere, and sometimes, and many “conjuncts” such as therefore, then, besides, otherwise, however, and instead.
signs of the reality of the distinction: (1) One clear indication that the distinction between closed- and open-class words is “real” (i.e. something that is objectively in the language, rather than something that grammarians have invented in describing the language) is the fact that the consonant [ð] appears initially only in closed-class words (e.g. the, this). (2) Another sign: “block language” (as used in newspaper headlines, notes, diaries, etc.) works largely by omitting closed-class words.(3) There also seem to be differences between languages as to the correlation between word classes and the closed- open-class distinction. According to the Wikipedia entry, “Closed class,” in Japanese, unlike English, pronouns are open-class and, unlike English once again, verbs are closed class.