When to use auxiliary verbs
To agree using so and neither
We use auxiliary verbs with so and neither to show agreement to something that has been said without having to repeat the same verb.
We use so + auxiliary verb + subject to agree to affirmative sentences.
- A: ‘I love jazz.’ B: ‘So do I‘
- A: ‘I’ll try again.’ B: ‘So will I‘
- A: ‘Tom has already finished.’ B: ‘So has Gina‘
We use neither/nor + auxiliary verb + subject to agree to negative sentences.
- A: ‘I’m not going to accept the invitation.’ B: ‘Neither am I.‘
- A: ‘I can’t concentrate’ B: ‘Nor can I.‘
- A: ‘I didn’t like the food’ B: ‘Neither did I.‘
- I couldn’t find the solution and neither could my wife.
If there is an auxiliary verb in the previous sentence, we use the same verb to agree. If there is no auxiliary verb in the previous sentence, we use do or did.
We use (Yes,/No,) + subject + auxiliary verb to show disagreement. If the verb in the previous sentence or clause is positive, we use a negative auxiliary and vice versa.
- A: ‘I’m not going to accept the invitation.’ B: ‘I am.‘
- A: ‘I didn’t like the food’ B: ‘I did.’
- I couldn’t find the solution but my wife could.
- A: ‘You will be here before midnight.’ B: ‘No, I won’t.’
To avoid repetition
We use auxiliary verbs to avoid repeating the same verb or verb phrase in a sentence.
If the first part contains an auxiliary verb, we use the same verb in the second part.
- I don’t like going to spas, but my wife does.
- I thought I had lost my keys, but I hadn’t.
- Tom can play the guitar and Tina can’t.
- She has never had to face such difficulties, but I have.
If the first part doesn’t contain an auxiliary verb, we use do or did in the second part.
- She shares your opinion, but I don’t.
- Some wanted to go on with the plan, but others didn’t.
In reply questions
We use reply questions to show interest or surprise. In the reply question we use the same auxiliary verb as in the previous sentence. If there is no auxiliary verb in the previous sentence, we use do or did. If the verb in the previous sentence is positive, the reply question is positive and if the verb in the previous sentence is negative, the reply question is also negative
- A: ‘Sarah hasn’t passed the exam.’ B: ‘Hasn’t she? I thought she would pass it.’
- A: ‘I won’t be able to be there.’ B: ‘Won’t you? That’s a shame.’
- A: ‘I can find the answer without using a calculator.’ B: ‘Can you? That’s amazing!’
- A: ‘She left without saying goodbye.’ B: ‘Did she? That’s very rude.’
To show emphasis
When we want to add emphasis to a verb, we often stress the auxiliary verb, if there is one, when we speak.
- A: ‘Why aren’t you going to the party?’ B: ‘I am going to the party.’ (We stress am)
However, we can’t do this in present and past simple affirmative sentences, because we don’t use an auxiliary verb in those sentences. What we do in those cases is use do/does (present) or did (past) before the main verb.
- A: ‘Why didn’t you go to the party?’ B: ‘I did go to the party.’
We normally show this emphasis when we want to contradict what somebody says.
- A: ‘Sally doesn’t speak Spanish.’ B: ‘She does speak Spanish. She lived in Spain for two years.’
- A: ‘You didn’t say sorry.’ B: ‘I did say sorry, but he didn’t hear me.’
- A: ‘You don’t want to save our marriage’ B: ‘I do want to save our marriage, but you don’t let me try.’
In question tags
We can use a question tag at the end of a sentence to ask for confirmation or to check information that we think is true. If the verb in the sentence is positive, we use a negative auxiliary verb in the question tag. If the verb in the sentence is negative, we use a positive auxiliary verb in the question tag.
- He is from Liverpool, isn’t he?
- You have done this before, haven’t you?
In the question tag we use the same subject that appears in the sentence, but always in the form of a personal pronoun: I, you, he, she, it, etc. We also use the same auxiliary verb as in the main verb in the sentence. If there is no auxiliary verb in the sentence, we use do or did in the question tag.
- Lisa went by train, didn’t she?
- The shops are closed today, aren’t they?
- You don’t have a car, do you?
There are some cases, when the question tag takes a special form:
- I am a bad person, aren’t I? (I am ⇒ aren’t I?)
- Open the window, will you? (Requests ⇒ will you?)
- Let’s go to the park, shall we? (Suggestions ⇒ shall we?)