1. Use A/An in front of nouns that are:

• singular (one)
• countable (can be counted)
• indefinite (not specific, not spoken about before)

2. Use A/An when you mean one, but you are not calling attention to the number.
Use ONE when you mean "only one" or want to call attention to the number or contrast it with another number.

3. Use A/An when you are telling that someone has a certain occupation.

4. Use THE to point out a definite person, place, thing or idea or one that has been mentioned or referred to before.

5. Use THE when the reader already knows which things you mean, or when there is only one possible item you are speaking about.

6. Use THE with things that are the only ones of their kind or class (the sun, the moon, the President of the United States, etc.)

7. Do not use THE with abstract nouns (freedom, art), when spoken of in a general way.
Do not use THE with months, days of the week or most holidays.
Do not use THE before personal names.
Do not use THE with the name of a language, a school subject or a field of study when spoken of in a general way.

8. Use THE with the names of period of history (ages, years, centuries) and the names of wars, battles, agreement, treaties, alliances.

9. Use neither A/An or THE with the specific names of continents, countries, cities, states, counties, mountains, lakes, islands, beaches, streets, avenues, parks.
Exceptions:

• groups of islands or lakes or mountain ranges
• names with plural forms
• names containing Union, United, Kingdom, Republic, Empire, or Dynasty.

A. Staben
College of Lake County Writing Center