The sentence

The difference between ‘a/an’ and ‘one’
1. We do not use a/an + noun and one + noun in the same way.
We use a/an to mean ‘any one’: I’d like a coffee, please.
We use one when we are counting: It was one coffee I ordered and not two.
2. We use one with day, morning, evening when we are telling a story:
One day, when I was working as a salesman, I received a strange telephone call.
Compare: I had to stay in bed for a day. (= any day, it doesn’t matter which)
I had to stay in bed for one day. (= one day and not two or more)
3. We use a/an or one with:
a. Whole numbers: a/one hundred, thousand. c. Money: a/one pound, dollar.
b. Fractions: a/one quarter, half. d. Weight, measures: a/one kilo, foot.
‘A/an’ for price, distance and frequency: ‘80p a kilo’
price/weight: 80p a kilo distance/speed: 40 km an hour
distance/fuel: 30 miles a (or to the) gallon frequency/time: twice a day
“A/an’ or zero with reference to illnesses: ‘a cold’
1. We always use a/an with these illnesses: a cold, a headache, a sore throat.
2. We can use or omit a/an with these:
catch (a) cold, have (a/an) backache/earache/stomach-ache/toothache.
3. We use no article at all with these plurals: measles, mumps, shingles.
4. We use no article with these: (high) blood pressure, flu, gout, hepatitis.
C. The definite article: ‘the’ (2)
‘The’ for specifying
1. When we use the, the listener or reader knows or can understand what we are referring
to.
We can make a reference ‘specific’ or ‘definite’ by means of:
- back reference: We stopped at a small village. The village was very pretty.
- the + noun + of: The life of Napoleon was very stormy.
- a clause: (= part of a sentence): The Jones I’m referring to is a colleague of mine.
- a context: The listener knows exactly what we are referring to from the context.
That’s why we say: It’s the postman, (Not *a postman*)
She’s gone to the butcher’s. (Not *a butcher’s*)
Running is good for the heart. (Not *a heart*)
2. We often say the cinema, the theatre, the supermarket, the bank, etc. even if we don’t
know exactly which: He’ gone to the cinema/the theatre/the supermarket/the bank.
3. We refer to the country, the mountains, the seaside even if we don’t know exactly
where:
We’re spending the weekend in the country/in the mountains/at the seaside.
4. We use the to refer to ‘one of a kind’: the earth, the sea, the sky, the sun, the moon, the
solar system, the planets, the galaxy, the universe: The earth doesn’t belong to us.
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