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What are auxiliary verbs? Auxiliary verbs are ‘helping’ verbs that are divided into two main categories; The ‘main auxiliaries’ that are; ‘do, have and be‘ and the ‘modal auxiliaries’ which are; ‘can, could, may, might, should, must, shall, will, would, ought to, need to and dare to‘. Dictionary definition of auxiliary verbs: ”accompanying another verb and typically expressing person, number, mood, or tense In; “I will go,” the word “will” is an auxiliary verb

Base form = do

Present form = do/does

Past form = did

Present participle/gerund = doing

Past participle = done

(1) We use to do to form interrogatives in English, that is, to make questions. This also includes question tags. ‘Do’ is used with normal verbs to form question tags (NOT with auxiliary or modal auxiliaries).  ‘Do’ is a general purpose verb for forming questions. 
  • Does Sally like to play the piano? Yes, she likes to play.
  • Did you do your homework yesterday?
  • Do you play soccer?
  • George writes poetry, doesn’t he? (question tag)
  • They go to school, don’t they? (question tag)

If a sentence starts with another auxiliary verb then the question tag following it must use the same verb as the auxiliary verb. Negative to positive or positive to negative.

(2) We use to do to be extra emotive or put more emphasis on the sentence. It is only used as an emphasis with ordinary verbs and not other auxiliary verbs. 
  • Did you do your homework last night? Yes, I did do my homework last night.
  • do not appreciate it when people speak loudly.
  • They do not have any idea about the situation at all.
  • By not using a contraction the verb ‘do’ is automatically used for emphasis.
(3) We use to do to negate, that is, to make sentences or questions negative. We don’t however, use ‘do’ to negate using other auxiliary verbs such as for example; have, be, can could etc.. Therefore it is wrong to say: ”I don’t can go” or ”She doesn’t must pass the exam”. We use ‘do’ to negate ordinary verbs NOT auxiliary verbs. 
  • don’t like tomatoes.
  • Do you look up to the sky at night? No, I don’t look up to the sky at night,
  • I don’t like maths.
  • Did you go out to the party last night?
  • Doesn’t he know anyone in the neighbourhood?
  • Did you do your assignment?

Note that in the last example the first ‘do’ in the sentence functions as an auxiliary verb and the second ‘do’ in the phrase functions as an ordinary verb. So ‘do’ works both as an auxiliary and an ordinary verb.

(4) We use to do as a shortened verb form or like a sort of ellipsis. We already assume the recipient knows what we’re talking about, so it’s not necessary to complete the whole phrase again. 
  • Did she play tennis last night? Yes, she did. (instead of; ‘yes, she did play tennis…)
  • Do you like playing rugby? No, I don’t(instead of; ‘no, I don’t play…)
  • Does he act strangely? Yes, he does. (instead of; ‘yes, he does act strangely)

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