In linguistics, a count noun is a noun that can be modified by a numeral and that occurs in both singular and plural forms, and that co-occurs with quantificational determiners like every, each, several, etc. A mass noun has none of these properties, because it cannot be modified by a numeral, cannot occur in plural, and cannot co-occur with quantificational determiners.
Examples
Below are examples of all the properties of count nouns holding for the count noun chair, but not for the mass noun furniture.
Some determiners can be used with both mass and count nouns, including "some", "a lot ", "no". Others cannot: "few" and "many" are used with count items, "little" and "much" with mass.
Grammatical distinction
The concept of a "mass noun" is a grammatical concept and is not based on the innate nature of the object to which that noun refers. For example, "seven chairs" and "some furniture" could refer to exactly the same objects, with "seven chairs" referring to them as a collection of individual objects but with "some furniture" referring to them as a single undifferentiated unit. However, some abstract phenomena like "fun" and "hope" have properties which make it difficult to refer to them with a count noun. Classifiers are sometimes used as count nouns preceding mass nouns, in order to redirect the speaker's focusaway from the mass nature. For example, "There's some furniture in the room" can be restated, with a change of focus, to "There are some pieces of furniture in the room"; and "let's have some fun" can be refocused as "Let's have a bit of fun". In English, some nouns are used most frequently as mass nouns, with or without a classifier, but also, less frequently, as count nouns
Some languages, such as Mandarin Chinese, treat all nouns as mass nouns, and need to make use of a noun classifier to add numerals and other quantifiers. The following examples are of nouns which, while seemingly innately countable, are still treated as mass nouns:
那个人吃完了 – "That unit person has eaten", "That person has eaten"
那三个人吃完了 – "Those three unit person' have eaten", "Those three people have eaten"
她有七本书 – "She has seven volume book", "She has seven books."
A classifier, therefore, implies that the object referred to are countable in the sense that the speaker intends them to be enumerated, rather than considered as a unit. Notice that the classifier changes as the unit being counted changes. Words such as "milk" or "rice" are not so obviously countable entities, but they can be counted with an appropriate unit of measure in both English and Mandarin. The use of a classifier is similar to, but not identical with, the use of units of measurement to count groups of objects in English. For example, in "three shelves of books", "shelves" is used as a unit of measurement. On the other hand, some languages like Turkish treats all the nouns, even the not so obviously countable entities as count nouns.
Pirinçler daha tam pişmemiş. – "The rice hasn't been cooked well yet"
Sütler hep yerlere döküldü. – "The milk has been spilled all over the floor "
Nehirlerinsuları çok güzel akıyor. – "The rivers' water flows very nicely"
Parasız kişiler için kitaplar dağıtıyorlar. – "They are distributing books for the people without money"
Even then, it's possible to use units of measures with numbers in Turkish, even with the very obviously countable nouns. Note that the Turkish nouns can't take a plural suffix after the numbers and the units of measure.
Beş bardak süt – "five glasses of milk"
İki kaşık dolusu pirinç – "two spoonfuls of rice"
Üç tane kişi – "three units of person"
Dört metrekare yer – "Four square meters of floor"