Word classes

Introduction to Word Classes

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:

They know a lot.

(The tick shows that this sentence is grammatically good.)

know can be replaced by other verbs.

They complain a lot.

They eat a lot.

They forget a lot.

But know can’t be replaced by a noun regardless of how similar their meaning might be. It can’t even be replaced by the noun knowledge, unless the sentence structure itself is changed.

They knowledge a lot.

They complaint a lot.

They eater a lot.

They forgetful a lot.

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:

He knew a lot about survival skills.

He knowledge a lot about survival skills.

His knowledge of survival skills was immense.

His knew of survival skills was immense.

Words of different classes have different effects even when their meanings are very similar.

How words are classified

Words are classified according to the work they do in a sentence.

Words that belong to the same word class cover the same range of jobs.

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.


When I arrived at their house, the big dog, which was called Rover, was barking loudly because it was lonely.


Warning about the term ‘part of speech’

Word classes are sometimes referred to as “parts of speech“, but this term should be avoided, because it’s 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.

Self-assessment on 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.


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