Definite & indefinite articles
It is almost impossible to discuss the noun phrase without referring at some stage to the class of words known as determiners, since more often than not a noun will occur with one or more words from this grammatical class. Determiners include articles (a/an, the) and quantifiers.
Determiners consist of a relatively small number of mainly grammatical items that change very little and tend to serve only one specialised function in a sentence. Unlike verbs, nouns and adjectives as word classes, it is difficult, if not impossible, to add any new words to the class of determiners and for this reason it is called a closed set.
What kind of words make up this limited set? The most instantly recognisable is probably thearticles namely the, a/an. In addition, there are others like: that, those, every, some, several, all, much, both, no, which can occupy the space before a noun.