An article is a word that acts as an adjective to modify a noun. However, adjectives, in general modify nouns through description, but articles are used to point out or refer to nouns. In other words, the articles are words that define a noun as specific or unspecific.
There are two types of articles in English language, definite and indefinite article.
The word ‘the’ is one of the most common words in English and it is the only definite article that is used to indicate a noun that is definite. Examples - The sun sets in the west, the man who knew too much, this is the story behind the screen.
The words ‘a’ and ‘an’ are indefinite articles. Each of these articles is used to refer a noun which is not a specific person, place, object or idea.
If the noun that comes after the article begins with a consonant sound, the article ‘a’ should be used as the appropriate indefinite article. Example – A man was standing alone near the closed doors, a tiger was roaring in the cage.
Similarly, the article ‘an’ should be used before a singular noun with a vowel sound. Examples – He is an artist of repute, I saw an eagle on the top of the hill.
It should be carefully noted that, the use of ‘a’ and ‘an’ do not depend on the spelling of the word it comes before, it depends on the pronunciation of the word. However, in most cases, ‘an’ is used before words that begin with vowels.
For example, the word ‘hour’ begins with the consonant ‘h’. But, since the ‘h’ is silent, the word has a vowel sound. Hence, it should be ‘an hour’. But, ‘a’ should be used, when ‘h’ is the first letter of a word and the first syllable is strongly pronounced. Example – ‘a history of Europe’. Similarly, when ‘h’ is the first letter of a word and the first syllable is weakly pronounced, ‘an’ should be ideally used, especially in British English. Example - ‘an historical occasion’ (hisTORIc), ‘an habitual liar’ (haBITual). However, these usages have now become old-fashioned.
Now, let us take the unique example of the word, ‘university’. It begins with the vowel ‘u’ But the ‘u’ is pronounced as if begins with the consonant ‘y’ (as in the word ‘you’) Hence, it should be, ‘a university’. But the word ‘umbrella’ also starts with ‘u’, but, it starts with the vowel sound ‘uh’. Hence, it should be, ‘an umbrella’.
The rule is applicable for acronyms also. Finally, if a letter is pronounced as a letter and it begins with a vowel sound it should be preceded with ‘an’. Example – He is an MA. Similarly, if a vowel letter, with a consonant sound, is pronounced as a letter, it should be preceded with ‘a’. Example – ‘He took a U-turn’.