The major word classes

It is common among non-grammarian to characterise the main word classes in terms of their meaning. While this is useful as a first step in familiarising oneself with nouns, verbs, adjectives and adverbs, there are more reliable ways of defining these classes. For more advanced definitions, see the following:

• The morphology of the major word classes
• The syntax of noun phrases

Verses like the following are often used to teach children what kinds of words belong to the word class noun.  

A noun's the name of any thing
like house or garden, boat or swing

This verse gives some indication of what a noun is by saying something about the characteristic meaning or reference of nouns. However, it only lists concrete 'things' as examples of nouns. In reality, many more words are nouns than those that refer to concrete things in the world. For example, words that refer to abstract concepts like democracy, irritation, scepticism, etc. are also nouns.

The following verse, like the one used to identify nouns, provides a first indication of what kinds of words are verbs, again by trying to capture one of their meanings:

Verbs tell something to be done:
To read or count or laugh or run.

The verse is limited in that it only includes words from one class of verbs, namely ones that refer to activities that are performed by animate individuals. Verbs that do not readily fit this characterisation include: enable, include, occur, and many others.

The following verse identifies adjectives as descriptive words, with a focus on the ones that denote properties of concrete objects.

The adjective describes a thing
like magic wand or bridal ring

A large number of adjectives that are used in academic writing are used to describe more abstract concepts. Thus, we are likely to find phrases like the following (with the adjective in boldface):

external causes of mortality
primary mortgage market

While adjectives are used to describe 'things', adverbs are often used to describe states and events, as the following verse indicates.

How things are done the adverbs tell,
like slowly, quickly, ill, or well.

The verse only captures one of many types of meanings that adverbs have. Adverbs may also be used to describe states and events in such terms terms as time (frequently, immediately, momentarily), degree (thoroughly, slightly, somewhat), contrast (alternatively, however), speaker attitude (surprisingly, unfortunately), and many others. In addition, adverbs are used to describe adjectives, e.g. in terms of degree (extremely dangerous), and other adverbs (very slowly).