7. Articles

Article 'a' is used before singular countable nouns beginning with a consonant sound.

Examples:

A lake, a dog, a teacher, a book, etc.

Exception:

Article 'a' is used before nouns which begin a vowel when written but make a consonant sound when spoken.

Examples: 

A European man, a university, a union, a unicorn, a uniform 

(The above nouns begin with vowels 'e' or 'u' but makes a consonant sound 'yoo'; hence we use 'a')

A one-rupee note.

(The noun begins with vowel 'o' but makes a consonant sound 'w'; hence we use 'a')