Definite or indefinite articles are often placed before a noun or noun phrase to indicate a general instance of that noun. Many nouns, however, do not require articles. In English, a zero article denotes a lack of an article preceding a noun or noun phrase. Non-countable nouns, proper names, and abstractions, as well as many locations and plurals, all take zero articles.
There are only three articles in English: "an," "a," and "the." The words "an" and "a" denote a general singular, countable noun, while "the" normally denotes a specific singular or plural countable noun. A countable noun is simply a noun which can have a number. For example, there can be five trees but not five airs. The word "air" is a non-countable noun, meaning it is a basic term for a general or intangible object. Non-countable nouns and plural countable nouns are some of the most common nouns which take zero articles.
For example, the sentence, "The diver has air," contains the nouns "air" and "diver." The word "diver" is a countable noun and refers to a specific person, so takes the definite article "the"; "air" is non-countable and so has a zero article. Conversely, the sentence, "That is a tree," contains a singular countable noun, "tree," and therefore must have an article. Changing the sentence to a plural results in "those are trees" and a zero article. If the speaker is speaking of a significant set of trees, however, the definite article remains in the plural: "Those are the trees."
Other nouns may take a zero article or a definite article depending on preference. For example, official titles may or may not have an article. Saying, "He is CEO of the company," and, "He is the CEO of the company," are both grammatically correct.
On the other hand, some abstract nouns may need articles if they are used in a specific instance. For example, "Love is fickle," does not require an article because in this sentence the concept of love is spoken of in general terms. Saying, however, "The love of a woman is fickle," gives a specific context to the love in question and therefore does need an article.
Certain countable nouns may take a zero article as well if they are being used for the function with which they are normally associated. For example, when saying, "She laid in bed," the definite article "the" is unnecessary. Saying, "She sat on the bed," however, requires the article. Similarly, saying, "He is in prison," requires a zero article, but "I went to a prison for lunch," requires an article because a prison is not normally a place where people go for meals. Only certain nouns follow this rule, however, and most still require an article regardless of their use.