last updated 9 April 2003
Knowledge of word classes can help explain why some words are interchangeable in the same sentence structure, and some need different structures.
Take the word know in the following sentence:
(The tick shows that this sentence is grammatically good.)
know can be replaced by other verbs.
But know canít be replaced by a noun regardless of how similar their meaning might be. It canít even be replaced by the noun knowledge, unless the sentence structure itself is changed.
However, although the noun knowledge is no use in this sentence, it is very useful in other sentences where the verb know does not work:
Words of different classes have different effects even when their meanings are very similar.
Words are classified according to the work they do in a sentence.
The jobs done by verbs, for example are quite different from those done by prepositions.
In this sentence, you can see how each word is classified by clicking on it.
Word classes are sometimes referred to as "parts of speech", but this term should be avoided, because itís misleading. "Speech" can be divided into many different 'parts', and many of these are phrases, not words, so they cannot be word classes or even members of word classes.
Each word class has a separate page, with a set of self-assessment exercises at the end. There is no self-assessment for word classes in general.