Nouns are either "count" or "non-count." A count noun represents something that can be counted:
A psychologist gave Joe a test.
Two psychologists gave Joe three tests.
A non-count noun, on the other hand, represents something that cannot be counted. In the following two sentences,knowledge, soccer, water, oxygen, and hydrogen represent things that cannot be counted.
Ø Knowledge about Ø soccer is growing.
Ø Water is made from Ø oxygen and Ø hydrogen.
Sometimes, a non-count noun is referred to as a "mass" noun. The term "mass" means something taken as a whole or something that is not divided into parts.
Identifying Non-Count Nouns
Unfortunately, it is not always possible to predict whether a noun is count or non-count. A dictionary will usually be helpful. But there are some categories of nouns that are often non-count. The three most general of these categories are illustrated below.
Abstractions: courage, nonsense, independence, evidence, advice, progress, information, employment
Everyday things: stuff, jewelry, clothing, traffic, furniture, money, equipment
Things to eat: butter, milk, beer, toast, salt
Some more specific non-count categories are the following.
Weather-related phenomena: sunshine, thunder, snow
Sports: football, racing, chess, golf
Substances: oxygen, sodium, ice
Fields of study: math, grammar, English, history
Expressions of Quantity with Non-Count Nouns
Often one wants to talk about some quantity of a non-count noun. English does this through the use of a phrase that has four components appearing in the order shown:
1. a word expressing the quantity
2. a count noun
3. the preposition of
4. the non-count noun
I ate one (1) piece (2) of (3) toast (4) for breakfast.
I hate several (1) pieces (2) of (3) toast (4) for breakfast.
Nouns That Do Double Duty
Some nouns that are non-count in some contexts can be legitimately used as count nouns in other contexts.
I'd like a glass of water (non-count) please.
God spoke and the waters (count) came together.
Most of us have long since learned to put up with snow (non-count).
The snows (count) generally come to this area in November.
Sugar (non-count) is sweet.
Chemically, sugars (count) are labeled with the suffix "-ose."
It is important to note that the number of non-count nouns which can be used also as count nouns, as in the above examples, is not fixed. As language evolves to cover experience, nouns that were once solely non-count can begin to be used also as count nouns:
Can you give me another couple of milks please. (= cartons of milk)
He's had too many beers, if you ask me. (= glasses of beer)