1. ONE as a Pronoun

A) We can use ONE instead of a noun phrase beginning with A, when it is  clear what  sort of thing we  are  talking about; e.g.

i) If you want a drink, I’ll get you one.
ii) Although she wasn’t a very clever woman, she looked and acted like one.
iii) The rooms were empty except for one at the second floor.

NOTE: We can’t use a plural form of ONE in these kinds of sentences; e.g.

INCORRECT: If you like mangoes, I’ll get you ones.
CORRECT: If you like mangoes, I’ll get you some.

INCORRECT: I have many books, mostly novels but ones on philosophy too.
CORRECT: I have many books, mostly novels but some on philosophy too.

B) We can use ONE or ONES instead of a countable noun when the noun comes  after an adjective; e.g.

i) I’ve had this car for a long time, and I’m thinking of buying a new one.
ii) I bought this computer for Rs 30,000. It’s quite a good one.
iii) This idea has become a very popular one.
iv) We have built many houses and are building new ones.

C) We can also use ONE or ONES instead of a countable noun in front of a  relative  clause or a prepositional phrase; e.g.

i) Of all the subjects, science is the one I love best.
ii) Could I see that shirt again, the one with wide stripes?

D) We can use ONE instead of a singular countable noun when the noun comes  immediately after any determiner except A; e.g.

I bought these dresses from Kolkata. That dress is different.
= I bought these dresses from Kolkata. That one is different.

i) I need to buy a new car. This one is too uncomfortable.
ii) He took the glasses and wrapped each one carefully.

NOTE: Don’t use THE ONE in front of OF + A NAME; e.g.

INCORRECT: This is my mug. That’s the one of Jane.
CORRECT: This is my mug. That’s Jane’s.

2. Pronouns ONE, YOU, WE, THEY when referred to ‘PEOPLE IN GENERAL’

We can use ONE, YOU, WE and THEY to refer to ‘people in general’, rather than to any particular people.

A) ONE/YOU/WE

We can also use ONE, YOU or WE for people in general and not referring to any one person in particular; e.g.

a) One should always get up early to stay healthy.
= You should always get up early to stay healthy.
= We should always get up early to stay healthy.

b) One usually needs to have woolen clothes in Shimla.
= You usually need to have woolen clothes in Shimla.
= We usually need to have woolen clothes in Shimla.

NOTE-I: We use the possessive determiner ONE’S and the reflexive pronoun ONESELF for ONE. We can also use YOU/YOUR/YOURS/YOURSELF and WE/OUR/OURS/OURSELVES in this use; e.g.

One wants only the best for one’s children.
=You want only the best for your children.
= We want only the best for our children.

There are things one can do to make oneself and one’s children happier.
= There are things you can do to make yourselfand your children happier.
= There are things we can do to make ourselves and our children happier.

NOTE-II: If ONE is the subject we must use ONE’S as its possessive  and ONESELF as the reflexive pronoun if required (not HIS and HIMSELF), and the verb is always singular; e.g.

INCORRECT: One always prefer to do his work himself.
CORRECT: One always prefers to do one’s work oneself.

INNCORRECT: One should do his duty himself.
CORRECT: One should do one’s duty oneself.

B) THEY

We can use THEY for people in general and not referring to persons in particular, such as an authority or an institution; e.g.

i) They found the body in the river.
ii) They are cutting down the trees.
iii) They’ve opened a new cinema in the town centre.

NOTE-I: We use THEY and THEM to refer back to the indefinite pronouns SOMEONE, ANYONE, EVERYONE if the number or gender is unknown.

i) If someone comes with a magazine for me, can you ask them to take it next door?
ii) I believe anyone can do anything if they really want, can’t they?

NOTE-II: We also use THEY to mention a saying; e.g.

i) They say that the camera never lies.
ii) They say that excess of everything is bad.an’t they?

NOTE-II: We also use THEY to mention a saying; e.g.

i) They say that the camera never lies.
ii) They say that excess of everything is bad.