A, an or one: counting and describing

Two jobs before a noun

A singular countable noun names one separate person or thing: a visitor, an island, one drawer. A/an is an article, a word before a noun. One is a number in one drawer, two drawers. Both can refer to one item, but only one is the number word.

• We hired a van. This introduces the vehicle, without making its number the main point.
• We hired one van, not two. The number matters.

Do not learn the false rule that a/an has nothing to do with number. It goes with a singular noun here. The difference is that one can make the count explicit or contrast it with another count.

You do not normally replace the article with one just to name a job. Her brother is one mechanic would need a special meaning or context, not the ordinary job description.

Known to whom?

A/an does not mean that you, the speaker, know nothing about the person or thing. You can introduce someone whom you know but have not identified to the listener.

• I spoke to a neighbour this morning. You may know exactly which neighbour you mean.

Nor does one automatically mean the one we both know. In We need one volunteer, you may not know who the volunteer will be. For the choice between introducing something and referring to an identifiable thing with the, see A, an or the?: introducing and identifying.

The sound of a, an and one

Choose a or an by the next sound. Use a before a consonant sound and an before a vowel sound. Look at the next spoken word, even when an adjective comes before the noun.

• It is an unusual design.
• It is a very unusual design.

In unstressed speech, a is /ə/ and an is /ən/. The number one is /wʌn/. It does not change to an before a vowel sound: one egg, one onion.

• We bought a one-way ticket. One begins with a /w/ sound, so the article is a.

In a one-way ticket, a is the article and one is part of the description one-way. This is not the same structure as two determiners before the noun, such as a one ticket. The complete sound and noun patterns are in A, an and some.

When the number matters

Use one in a count, or when you contrast a single item with two or more. Only can make the limit explicit. One more means an additional single item.

• The box contains one brush and three cloths.
• I need only one signature.
• Could you read the address one more time?

If the number is not the focus, a/an is possible in many of these situations. Please bring a chair and Please bring one chair can ask for the same item. One puts the number into the request. Neither sentence is wrong just because the other is possible.

Do not turn this into a rule that every sentence with one must contain not two. The contrast can be understood, or you can simply be counting.

One member compared with another

You can use one to pick out a particular member of a group or pair and compare it with another. This need not mean that there is only one item in the whole group.

• One window faces the road; another faces the garden.
• One shoe is wet, but the other is dry.

Use the other when you mean the second of a known pair. Another can mean a different or additional member. Their full patterns are in Another, other and others: another day, other plans, the others.

Picking something out for emphasis

You can also use one to draw attention to a particular person or thing, rather than give a neutral description. Here emphasis means making a point stand out.

• One feature I value is the large keyboard.
• If there is one task I dislike, it is sorting old receipts.

These sentences pick out a feature or task. They do not claim that no other feature matters or that you dislike no other task. A/an would give a less emphatic description in these patterns. This is an extension of the basic counting contrast, not a reason to put one before every noun.

One day and one evening: later reading

In a story, one day, one evening and similar phrases can name a particular occasion without giving its exact date. You are not necessarily contrasting one day with two days.

• One evening, the lights went out during dinner.

You can also use one day for a future time that is not yet decided.

• One day, I would like to learn to sail.

Context matters: one day can name a duration or an occasion. It does not always mean a future dream.

Large numbers and rates

Both a hundred and one hundred can mean 100. Cambridge's number reference presents a hundred, a thousand and a million as the normal choices, with one hundred, one thousand and one million used to emphasise the number.

• The hall has about a hundred seats.
• The total is one hundred euros, not two hundred.

Do not add a second article: say a hundred or one hundred, not a one hundred in an ordinary count.

A/an has another job in prices, speed and frequency: it can mean per, or for each. One is not a direct replacement with the same rate meaning.

• The locker costs three euros a day. Three euros for each day.
• The pump moves forty litres an hour. Forty litres for each hour.

Three euros one day can refer to a price on a particular occasion, but it does not express the same per-day rate. Keep the meaning in view before calling a different sentence wrong.

One of a group

Use one of when you mean a single member of a group. The group noun is plural. It has a determiner such as the, these or my. You can also use an object pronoun such as us or them after of.

• One of these drawers is empty.
• One of us has the spare key.

When the whole subject is one of + group, use a singular verb: One of these drawers is empty, not are empty. You are making a statement about one drawer. This is the simple main-clause pattern; it is not a rule for every later verb inside a longer relative clause.

Do not confuse one of the drawers with a drawer. Both name a single item, but the first explicitly selects it from a group. You cannot replace one by a or an directly before of.

One without a following noun

An article cannot normally stand alone in place of a noun. When a singular countable noun is understood, you can use one instead. This one substitutes for a noun; it is not always a stressed number contrast.

• Have you got a torch? I need one for the cellar.
• My raincoat is torn, so I need a new one.

A new one is correct: a is the article and one stands for raincoat. Do not confuse it with a one raincoat, where one is a number before the noun. See One and ones: a smaller one, the ones by the door for the full substitution patterns and their limits with uncountable nouns.

What comes next

A, an and some covers sounds, noun types and amounts; A, an or the?: introducing and identifying covers identifying the reference; One and ones: a smaller one, the ones by the door covers replacing a noun; Another, other and others: another day, other plans, the others covers different and additional members. For the wider noun patterns, see Countable and uncountable nouns: a bag, some luggage and Singular and plural nouns: one cup, two cups.