A determiner is a word that is placed in front of a single noun or noun phrase (attributively). It cannot be used predicatively, that is after the noun. A determiner may or may not be used before plural nouns.
A determiner also precedes other adjectives if the adjectives are used to describe the noun. Determiners and most adjectives come in front of a noun, and often thought of as being the same. This is not so, as they do not belong to the same category. Only some determiners, but certainly not all determiners, act like adjectives. Unlike an adjective, a determiner is usually needed in a sentence as a modifier. Both determiners and adjectives serve the same function of modifying a noun by giving more information. But they also differ; for example, a determiner indicates whether the noun is a specific one or expresses the position of the noun in relation to the writer or speaker, whereas an adjective describes the quality of the noun.
There are four types of determiners: articles, demonstratives, possessives, and quantifiers to indicate the certainty, location, ownership, and quantity of the noun. Determiners may be divided into definite determiners and indefinite determiners.
Determiners and quantifiers are useful in letting us know more about the noun that they refer. For example, this thing identifies something that is close at hand; her wig means the wig belongs to her; another donkey tells of one more donkey; and a few marbles refers to a small number of marbles.
Most of the words used as determiners are also used as pronouns. However, the possessive determiners (my, your, his, her, its, our, and their) and a few other determiners cannot be used as pronouns: the, a, an, every, no, and other. To use the latter ones as pronouns, use one (instead of a or an), each (instead of every), none (instead of no), and others (instead of other).
A quantifier is a word or phrase that expresses the quantity of a noun by indicating how many things there are or how much of something there is. It precedes and modifies the noun, which can be a countable noun, or an uncountable noun. Examples of quantifiers are: all, many, much, most, one, some, a few, and a lot of, etc.