Articles (a / an / the) are the most used words in the English language. They carry implied meanings and can change the entire context of a sentence.

Look at these two sentences:
A boy with blonde hair came up to the police officer to ask for help.
The boy with the blonde hair came up to the police officer to ask for help.

The first sentence uses an indefinite article. This means that the boy might have been the only boy at the scene, or he may have been one of many boys with blonde hair at the scene.
The second sentence uses the definite article tells us that there was only one boy with blonde hair at the scene. The may have been other boys, but they had hair that was not blonde.

What is a definite article?

Definite means specific, only one, particular. In other words, it is limited in scope and can be defining.

Looking again at the above examples, we notice that the first sentence uses a modifying phrase (with blonde hair) to describe the boy. The second sentence uses the same modifying phrase to identify the boy. By identifying the noun, we make it definite and therefore need a definite article (the)

There are many ways to make a noun definite, or to identify it. Here are some:

  • An identifying adjective clause:

    The man who lives next door is a doctor.—as we identified the man, we need to show that there is only one many being talked about and thus add the the.
    Here is a similar sentence: A man, who lives next door, shouted out his window.—in this sentence we are using a non-identifying adjective clause and so are not identifying the man. We are only describing his location. We do not know who he is. There are probably other men who live in the same apartment/house. We do not know which one of them did the shouting.

  • A prepositional phrase:

    The girl with the braided hair kept looking at Steve. In this sentence we can understand that there are several girls, but only one has braided hair. She is looking at Steve. Yet, A girl with braided hair kept looking at Steve is not identifying the girl (there might be other girls with braided hair there), but merely describing her, saying what she looks like.

  • A possessive:

    Five of the cars were red and three were black.— Here we identify the whole group (of cars). Five of them (8) were red. Likewise, The City of London is in Britain limits the city to only one, which belongs to London, or is named London.
    However, if we use of with collective nouns, we can use definite or indefinite, depending on the situation: Larry gave her a box of chocolates as a gift. —She finished the box of chocolates in a couple of hours.

  • Only one:

    if there is only one sample of the noun, it is definite—The sun is hotter near the equator than in the northern regions. In a small town with only one hospital—The hospital is being renovated. However, keep in mind that the Earth is a planet in the solar system, but we live on Earth, the name of a place (like a city or country).

  • Second mention:

    After a noun has been introduced as an indefinite noun, it then becomes definite when mentioned again because now we know which one it is—A dog ran up to a child to play, but the child was afraid of the dog. Likewise, a second mention, but using a synonym, also takes the definite—Jack bought Tracy a rose and a daisy. She was so happy with the flowers that she gave him a kiss.

  • With superlative adjectives:

    the first, the best, the most, the most expensive, the tallest, etc.—Keith was always the tallest kid in his class.

What is an indefinite article?

Indefinite, then, suggests any. If there are several options and we do not identify one in particular, then we have an indefinite noun.

  • A friend of mine came over for coffee last night.—I have many friends, and I did not specify which one of these came over. It could have been any one of my friends. To make this definite, however, I would use my friend’s name (Tom). If I say My friend came over last night, this would suggest I have only one friend. (We do not use the together with an adjective pronoun—The my friend… is incorrect).
  • Shelley bought a new coat for the winter.—There are many new coats available at the store. We can’t know which one she bought. Shelley bought the winter coat that was on sale at Macy’s.

When should we use a, an, the, or no article?

  • Use a before a word that begins with a consonant (b, c, d, f, g, ….), or a consonant sound (useful sounds like yoosful). Use an before a word that begins with a vowel (a, e, i, o, u) or a vowel sound (MIT sounds like em I tee).
    An MIT Engineering degree is a useful credential to have. An engineer with this degree can get a good job.
  • Use a/an with singular countable nouns (a chair, a system, a fear (there are many types of fear)
    Use the with singular or plural countable nouns (the chair(s), the system(s), the fear(s))
    Decide which one to use based on definite or indefinite use
    We bought a new leather chair for the office. The old chair was broken.

    Do not use an article with countable plural nouns (Pencils are cheaper than pens, so we bought a few packsa here goes with few, not with packs) (a little, a bit, a couple of, etc.)

  • Use the with singular uncountable nouns (the water, the money, the light) when that noun is definite.

What would you like to drink? I’ll have the water = there’s a choice between water and soda etc. (I’ll have a water = a glass/bottle of water—the glass/bottle is understood and can be omitted, but the a does not go with water here.)

Do not use a/an with noncountable nouns: a money, a time, and a water are all incorrect. To use a/an with these, add a quantifier—an amount of money, a bit of time, a liter of water.

  • Use definite or indefinite article with an object/tangible feeling, but use no article when that same noun is an activity or concept:
    The will be a meeting for all parents at the school—the building
    Haley will graduate from school next month—school here is the concept/activity of studying
    Some people believe that love conquers all—the concept
    The love a parent has for a child is limitless—a tangible feeling
    Heather invited her boyfriend’s parents to dinner—activity
    They said that the dinner was delicious—the food
  • Classifications:
    Both articles, or no article can be used for generic nouns:
    A lion eats once a week.
    Lions eat once a week.
    The lion eats once a week.

All three examples mean the same if we are speaking about lions in general as an animal that behaves this way. The most common use is the third (plural noun), though the definite article can be used when we are speaking about animals in general to emphasize the lion (class).

We generally use the definite article or no article to speak about a class of transportation mode— The cheapest way to get to New York from Toronto is by bus. —here, bus is the method, concept; Taking the bus is the cheapest way to get to New York from Toronto.— here, taking the bus is the method, the bus is the object of the verb taking and is a generic noun.

Use the definite pronoun with collective nouns: the police, the military, the unions, etc.

  • Do not use articles with other adjectives that are preceded by a pronoun (my, his, some, that, etc.):
    He asked me if he could borrow the my car—incorrect (do not need the the)
  • Do not use articles with proper nouns: Bob, Dr. Smith, President Obama, etc.

Do not use articles with country, city, town, state, or province names.
Exception: with plural-named countries and republics, use the:
The United States, the UK, the Philippines, the Netherlands, etc.
Example: China has a growing economy.; The treaty was signed by the People’s Republic of China.

Use the definite article with names of rivers, oceans, seas, mountain ranges, forests, gulfs, peninsulas, deserts, points on the globe, and geographical areas:
The Thames River, the Nile; The Pacific Ocean, the Atlantic; the Mediterranean Sea, the Caribbean; the Himalayan Mountain Range, the Rocky Mountains; the Black Forest; the Gulf of Aden; the Sinai Peninsula; the Sahara Desert; the North Pole, the Equator; the Middle East, the American Midwest.
Exception: Individual mountains (with the word Mount before them) do not take an article (Mt. Fuji, Mt. Everest)
Do not use article with directions: northwestern Canada, We travelled south-east, etc.

Do not use an article with the names of streets, lakes, bays, continents, islands, sports, academic subjects, languages:
Main Street; Lake Louise; Hudson Bay; Asia; Hawaii; baseball; history; French

Groups of lakes take a definite article (the Great Lakes)
Groups of Islands take a definite article (the Canary Islands)

Be careful with languages and people: I don’t speak French.; The French know how to eat well.
Some languages and people share the same noun. The language does not take an article, but the people do. (Japanese, Spanish, Chinese, English, etc.) German refers to the language, the Germans refers to the people.

  • Periods of time: specific periods take a definite article—the 1960s; the golden years of television; the dawn of man; the 16th century.
    Set amounts of time take a definite or indefinite indefinite article: a decade, a century, the year, etc.

Special cases: in the morning, afternoon, evening, but at night (no article with night)
Prepositions of place: if you refer to a relative place/time, use the definite article (in the middle of…, in the front of…); if you refer to a relative location, do not use an article (in front of…, behind, next to…)— The teacher stood in the front of the room…. the teacher stood in front of the students.