Words are often left out in a sentence to avoid repetition. Sometimes we leave out words because the meaning can be understood without them. This is called ‘ellipsis’.

Replies
Ellipsis is common in replies in which we usually avoid repeating information that has just been given.

‘Who did this?’ ‘John.’ (More natural than John did this.)
‘How many players were there?’ ‘Five.’ (More natural than There were five players.)
‘What time are you coming?’ ‘About nine’. (More natural than I am coming about nine.)

Coordinate structures
Words that repeat information are usually dropped in coordinate structures.

He is poor but honest. (= He is poor but he is honest.)
You can have tea or coffee. (= You can have tea or you can have coffee.)

Informal speech
In informal speech, words at the beginning of a sentence are usually dropped if the meaning is clear.

‘Had dinner?’ (= ‘Have you had dinner?’)
‘Seen Mary?’ (= Have you seen Mary?)

At the end of noun phrases
Nouns are often dropped after adjectives or determiners.

‘Do you want large eggs?’ ‘No, I will have small.’ (= ‘I will have small eggs.’)

At the end of a verb phrase
At the end of a verb phrase, auxiliary verbs are often used alone instead of full verbs.

‘I haven’t got the letter.’ ‘I haven’t either.’ (= I haven’t got the letter either.’)

The same structures are possible with non-auxiliary be and have.

I thought that she would be interested, but she wasn’t. (= She wasn’t interested.)

Infinitives
Instead of repeating a whole infinitive, we sometimes use only to.

‘Are you and John getting married?’ ‘We hope to.’ (= We hope to get married.)

Sometimes a whole infinitive is left out.

Come when you want. OR Come when you want to. (= …when you want to)