Articles are the most frequent words in English and among the last a learner gets right. They carry almost no meaning of their own, they are unstressed and easy to miss in speech, and a sentence without them is still perfectly understandable. That combination is exactly what makes them persistent: nothing breaks when you get them wrong, so the error never corrects itself, and it goes on marking your English as non-native long after your vocabulary and your tenses have stopped doing so.
There are three choices, not two
English grammar offers two articles, and most courses stop there. In practice you are choosing between three options every time you use a noun, because leaving the article out is itself a grammatical choice with its own meaning, usually called the zero article. Learners who think there are only two options never consider the third, and so they insert something wherever a noun appears.
I bought a book.
I bought the book.
I bought books.
The three lines above are all correct, and each says something different. The first introduces one item the listener has not heard of before; the indefinite article marks the noun as non-specific, one of a kind rather than a particular one. The second refers to an item both speaker and listener can identify, which is what the definite article does. The third makes a general statement about more than one item, with no article at all. The difference between them is not politeness or formality, it is a claim about what your listener already knows.
The rules for a and an: sound, not spelling
The indefinite article has two forms, and the choice between them is decided by the first sound of the word that follows, never by the first letter. This trips up careful learners more than careless ones, because a careful learner looks at the written word.
a university, a European city, a one-way street
an hour, an honest answer, an MBA
Every item in the first line above begins with a vowel letter, and yet each takes the form used before consonant sounds, because each begins with a consonant sound the moment you say it aloud: the first two open with the gliding sound made by raising the tongue towards the roof of the mouth, and the third with the one made by rounding the lips. Every item in the second line begins with a consonant letter and takes the other form, because the first sound is a vowel sound: in the first two the opening letter is silent, and in the third the letter is pronounced as its name, which starts with a vowel. Say the word before you choose, and the rule never fails.
The same sound-before-spelling principle governs several other decisions in English, which is why the reference page on letters that are written but never spoken is worth keeping to hand. Note also that the indefinite article is singular only. It cannot appear in front of a plural noun, and it cannot appear in front of an uncountable noun, because both of those describe quantities rather than single items.
When to use the definite article
The definite article makes one claim: that the listener can work out which one you mean. Everything else follows from that. It is used with singular and plural nouns alike, and with uncountable nouns too, because identifiability has nothing to do with number.
Close the door, please.
The report you sent has arrived.
She is the tallest person in the office.
The Danube runs through four capital cities.
Each line above gives the listener a different route to the identification. In the first, the situation does it: there is one obvious door in the room. In the second, the words after the noun do it, narrowing the field to one. In the third, the superlative does it, since there can only be one tallest. In the fourth, the noun is simply unique in the world, and a small set of geographical names behaves this way. The most common route of all is not shown above, and it is mentioning the noun a second time: the first mention takes the indefinite article, every mention after it takes the definite one, because by then the listener knows which one you mean.
When to use no article at all
The zero article is the option learners forget, and it covers a great deal of ordinary English. Use no article when you are speaking in general terms about a plural noun or an uncountable noun.
Water freezes at zero degrees.
Cats sleep for most of the day.
She studies medicine at university.
We had breakfast at eight.
None of the four lines above refers to a particular item; each makes a general statement, and adding the definite article to any of them would change the meaning by narrowing it to something specific. The last two show a further group worth learning as a set: certain everyday nouns naming meals, institutions and forms of transport drop the article when you mean the ordinary activity rather than the physical building or object. Whether a noun can take the zero article at all depends on whether it is countable, so the reference table showing which everyday nouns take a plural and which never do is the companion piece to this section, and the list of plurals that refuse to follow the rule covers the forms that look singular but are not.
If your first language has no articles
Thai, Chinese, Japanese, Korean, Russian, Polish and many other languages have no articles at all. If you speak one of them, the difficulty is not that the rules are complicated, it is that you are being asked to make a distinction your language never requires you to think about, several times in every sentence, at conversational speed. The information carried by an English article is usually supplied in those languages by word order, by context, or by nothing at all, because the listener works it out.
That produces a specific and very recognisable pattern: articles are omitted rather than confused. The correction is not to memorise more rules but to build the habit of asking one question every time you produce a singular countable noun, since that is the case where English never allows nothing. If your language does have articles, your error will be the opposite one, transferring your own usage into English and putting the definite article in front of general plural statements where English wants none.
How to pronounce the definite article
The definite article has two pronunciations, and they follow the same sound rule as the indefinite one. Before a consonant sound it is reduced almost to nothing, with the neutral unstressed vowel English uses everywhere in weak syllables. Before a vowel sound the vowel lengthens and the word is easier to hear. Speakers also use that longer form deliberately before any word at all when they want to emphasise it, which is why you will occasionally hear it before a consonant.
This matters less for being understood than for understanding others: the reduced form is so short in fast speech that learners listening for it often do not hear it, conclude the speaker omitted it, and take that as permission to omit it themselves.
The mistakes that cost marks
Four errors account for most of the damage in exam writing. The first is omitting the article in front of a singular countable noun, which is never correct in English and is the commonest error among speakers of article-less languages. The second is the reverse, putting the definite article in front of a general plural or uncountable statement, which turns a general truth into a claim about specific items. The third is choosing the indefinite form by letter rather than sound. The fourth is using the indefinite article with a plural or uncountable noun, which cannot happen at all.
Examiners treat all four as grammatical accuracy rather than style, and because articles appear so often, a consistent error with them affects a great many sentences on the page. That frequency cuts both ways: it makes the error costly, and it makes practice efficient, because every paragraph you write gives you a dozen chances to rehearse the choice.
What to do this week
The three pieces here that go with this page are the table of all four choices, the lesson on letting the sound decide the form, and the practice set that makes you leave the gap empty when nothing belongs there. Then take something you have already written and mark only the singular countable nouns. Every one of them needs an article or a word doing an article-like job, and for most learners that single pass finds more errors than any exercise. Then read a page aloud and listen for the reduced form in your own speech; if it is not there, your listener will not hear it either. Everything on this site is grouped on the page that sorts material by level and on the page that sorts it by subject.