There are three articles in the English language: the, a, and an. These are divided into two types of articles: definite (the) and indefinite (a, an). The definite article indicates a level of specificity that the indefinite does not. “A solar panel” could refer to any solar panel; however, “the solar panel” is referring back to a specific solar panel.

Thus, when using the definite article, the writer assumes the reader knows the identity of the noun’s referent—because it is obvious, because it is common knowledge, or because it was mentioned in the same sentence or an earlier sentence. Use of an indefinite article implies that the reader does not have to be told the identity of the referent.

There are also cases where no article is required:

• with generic nouns (plural or uncountable): cars have accelerators, happiness is contagious, referring to cars in general and happiness in general (compare the happiness I felt yesterday, specifying particular happiness);
• with many proper names: Sabrina, France, London, etc.

Watch this quick introduction to indefinite and definite articles and the difference between the two:

Indefinite Article

The indefinite article of English takes the two forms a and an. These can be regarded as meaning “one,” usually without emphasis.

Distinction between a and an

You’ve probably learned the rule that an comes before a vowel, and that a comes before a consonant. While this is generally true, it’s more accurate to say that an comes before a vowel sound, and a comes before a consonant sound. Let’s look at a couple of examples with a:

• a box
• a HEPA filter (HEPA is pronounced as a word rather than as letters)
• a one-armed bandit (pronounced “won. . . “)
• a eulogy (pronounced “yoo. . . “)

Let’s try it again with an:

• an apple
• an EPA policy (the letter E read as a letter still starts with a vowel sound)
• an SSO (pronounced “es-es-oh”)
• an hour (the h is silent)
• an heir (pronounced “air”)

Definite Article

The definite article the is used when the referent of the noun phrase is assumed to be unique or known from the context. For example, in the sentence “The keen astronomy student was looking at the moon,” it is assumed that in the context the reference can only be to one keen astronomy student and one moon.

The can be used with both singular and plural nouns, with nouns of any gender, and with nouns that start with any letter. This is different from many other languages which have different articles for different genders or numbers. The is the most commonly used word in the English language.

Word Order

In most cases, the article is the first word of its noun phrase, preceding all other adjectives and modifiers.

The little old red bag held a very big surprise.

There are a few exceptions, however:

• Certain determiners, such as all, both, half, double, precede the definite article when used in combination (all the team; both the girls; half the time; double the amount).
• Such and what precede the indefinite article (such an uproar; what a day!).
• Adjectives qualified by too, so, as and how generally precede the indefinite article: too great a loss; so hard a problem; as difficult an exam as I have ever sat; I know how demanding a professor she is.