1.

You use a or an when you are referring to someone or something for the first time or when people may not know which particular person or thing you are talking about.

2.

You use a or an when you are referring to any person or thing of a particular type and do not want to be specific.

3.

You use a or an in front of an uncount noun when that noun follows an adjective, or when the noun is followed by words that describe it more fully.

4.

You use a or an in front of a mass noun when you want to refer to a single type or make of something.

5.

You use a in quantifiers such as a lot, a little, and a bit.

6.

You use a or an to refer to someone or something as a typical member of a group, class, or type.

7.

You use a or an in front of the names of days, months, or festivals when you are referring to one particular instance of that day, month, or festival.

8.

You use a or an when you are saying what someone is or what job they have.

9.

You use a or an in front of the names of artists to refer to one individual painting or sculpture created by them.

10.

You use a or an instead of the number 'one', especially with words of measurement such as 'hundred', 'hour', and 'metre', and with fractions such as 'half', 'quarter', and 'third'.

11.

You use a or an in expressions such as eight hours a day to express a rate or ratio.

·- Of.

II. A ·prep In; on; at; by.

III. A ·- An expletive, void of sense, to fill up the meter.

IV. A ·- A barbarous corruption of have, of he, and sometimes of it and of they.

V. A ·- An adjective, commonly called the indefinite article, and signifying one or any, but less emphatically.

VI. A ·- In each; to or for each; as, "twenty leagues a day", "a hundred pounds a year", "a dollar a yard", ·etc.

VII. A ·prep In process of; in the act of; into; to;

VIII. A ·- The name of the sixth tone in the model major scale (that in C), or the first tone of the minor scale, which is named after it the scale in A minor. The second string of the violin is tuned to the A in the treble staff.

IX. A ·- The first letter of the English and of many other alphabets. The capital A of the alphabets of Middle and Western Europe, as also the small letter (a), besides the forms in Italic, black letter, ·etc., are all descended from the old Latin A, which was borrowed from the Greek Alpha, of the same form; and this was made from the first letter (/) of the Phoenician alphabet, the equivalent of the Hebrew Aleph, and itself from the Egyptian origin. The Aleph was a consonant letter, with a guttural breath sound that was not an element of Greek articulation; and the Greeks took it to represent their vowel Alpha with the a sound, the Phoenician alphabet having no vowel symbols.