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Word Classes is a more modern name for what were more traditionally called Parts of Speech. Each is a set of words. Each set performs a separate function in a sentence. The members of some sets are distinguished by the way in which they are inflected, or how they can change their shapes, usually at the ends of the words. (For example, nouns are the only class in English that show the plural by adding 's' - and not all nouns do this.)
Word classes fall into two main groups:
- Words with meaning, sometimes called Form Words, Semantic words or Lexical words. Semantic Words are the words that carry most of the information in a sentence. They can be divided into four main groups: nouns, verbs, adjectives and adverbs.
- Function Words have less meaning than semantic words, but do useful jobs in language. They are the 'little words' that act as the glue, or connectors, inside a sentence. Without them, the Semantic words do not make as much sense. They include articles, prepositions, conjunctions and pronouns.
As a general rule, Function words form closed sets - i.e. new function words do not often enter the language. In contrast, Semantic words form Open Sets - i.e. new ones are constantly being developed. When an inventor produces a new product, or a scientist makes a new discovery, we need new words to describe or name them. And in slang, of course, new words are invented all the time - some of which become part of the formal language in time.
- Another group, the interjections, exists which is hard to classify as 'semantic' or 'function'. This is largely because interjections, being the sort of exclamations that are emitted as sudden expressions of strong feelings, do not readily form grammatical links with other words. They are mostly fairly simple, short words (or expressions) which stand on their own. Examples are "gosh! wow! My goodness! Ow! Ouch!" etc.
Quirk groups interjections with numerals as word-classes that are hard to analyze: interjections because they are almost 'pre-linguistic' in their primitive nature, and numerals because "they constitute a miniature syntax of their own, within the larger syntax of the English language." Neither interjections nor numerals are easy to pin down as either closed sets or open sets.
Grammarians (students of grammar) sometimes argue about how many classes of words there are. The number, for English, usually falls between 8 and 11. Don't worry about the finer details of a word class hierarchy, unless you are a student of linguistics or curious about it.
One confusing thing about Word Classes is that the same group of letters ('word') can be used in different ways. For example, you can back your car (where back is used as a verb); you can hurt your back (noun); you can go back (adverb); or you can look out of the back window (adjective). As can be a preposition (she was as good as gold) and a conjunction, as you can see. These distinctions are important if you are making a study of language, or learning a foreign language. At times, they do not matter if you are simply trying to acquire a rough knowledge of Word Classes.
For further reading, you may want to look at the Grammar course book list.