Short answers: Yes, he can, etc

Answers are often grammatically incomplete, because they do not need to repeat words that have just been said. A common ‘short answer’ pattern is subject + auxiliary verb, together with whatever other words are really necessary.

• ‘Can he swim?’ ‘Yes, he can.’ (More natural than Yes, he can swim.)
• ‘Has it stopped raining?’ ‘No, it hasn’t.’
• ‘Are you enjoying yourself?’ ‘I certainly am.’
• ‘Don’t forget to write.’ ‘I won’t.’
• ‘You didn’t phone Debbie last night.’ ‘No, but I did this morning.’

We use do if there is no other auxiliary.

• ‘She likes cakes.’ ‘She really does.’
• ‘That surprised you.’ ‘It certainly did.’

Non-auxiliary be and have can be used in short answers.

• ‘Is she happy?’ ‘I think she is.’
• ‘Have they any money?’ ‘Yes, they have.’

Short answers can be followed by tags ( (see here).

• ‘Nice day.’ ‘Yes, it is, isn’t it?’

Note that stressed, non-contracted affirmative forms are used in short answers.

• Yes, I am. (not Yes, I’m.)

note

For similar structures, (see here) (reply questions), (see here) (question tags) and (see here) (ellipsis).