Many scholars recognize the category of definiteness/mdefiniteness (article determination). Though the article is used as the morphological marker of the noun, it can hardly be treated as a word-morpheme. The position of the article may be occupied by other words (demonstrative and possessive pronouns, etc.). Words, which have a distribution including the article position, are called determiners. The role of determiners is to specify the range of reference of the noun by making it definite or indefinite.
Meaningful absence of the article (zero article), presupposes generalization.
In discussing the use of the articles it is essential to distinguish between specific, or particular reference, and generic reference (R.Quirk et al.):
Tlhe telephone is broken. (Specific reference)
The telephone is useful. (Generic reference)
The distinctions, which are important for countable nouns with specific reference, disappear with generic reference:
1) A telephone is useful.
2) The telephone is useful.
3) Telephones are useful.
The article plays an important role in structuring information. It is one of the means of distinguishing between facts already known (the theme) and new information (the rheme). The definite article is the marker of the theme, the indefinite article is the marker of the rheme.
Certain determiners (articles, demonstrative pronouns) can be used to show that a noun phrase is referentially equivalent to a previous noun-phrase. In such cases the article expresses co-reference, which is one of the means of sentence-connection.