If it is possible, we avoid the repetition of a word or phrase that has been used before. One way of doing this is to use a general purpose substitute word like it, that, one, do, there and so.

She folded the paper and put it in the envelope. (More natural than She folded the paper and put the paper in the envelope.)

‘Do you think that he will come?’ ‘I think so.’ (= I think that he will come.)

‘How about a drink?’ ‘I would like that.’ (= I would like a drink.)

‘What sort of house are you looking for?’ ‘One with a big garden.’

‘Let’s meet at the club.’ ‘OK see you there.’ (= See you at the club.)

Pronouns are the most common substitute words. They can substitute for nouns or noun phrases. They include personal pronouns (e.g. he, she, they), reflexive pronouns (e.g. himself, herself, itself), possessive pronouns (e.g. my, his, her), relative pronouns (who, that, which), demonstratives (e,g. this, that, these and those), interrogatives (e.g. what, who) and indefinite pronouns (e.g. somebody, anybody).

Jane was happy because she had won a prize. (NOT Jane was happy because Jane had won a prize.)

The old woman talks to herself all the time. (NOT The old woman talks to the old woman all the time.)

Have you ever met the people who live next door?

I would like something to eat.

We can use do so and do it / that as substitutes for a verb and words that follow.

‘Can you repair my computer?’ ‘I will do it at once.’ (= I will repair your computer at once.)

I asked him to help me but he wasn’t willing to do so.