do as auxiliary verb
Ali Almalki from Oman writes:
  What is the difference in use between I believe and I do believe
Hamid Hakim from Algeria also asks:
What's the difference if I say I did sleep last night instead of I slept last night?
Roger Woodham replies:
do / does
As you know, we normally use do or does + infinitive to form questions and in negative sentences in the simple present (does for the third person singular, he, she, it and do for all other persons, I, you, we, they):
Do you like music? - Yes I do.
Does Henry? - Yes, he does.
What kinds of music do you like? - I quite like reggae, but I don't care for garage very much.
Henry likes garage, but he doesn't get very excited about R & B.
As you can see from the above examples, we also use do and does in shortened verb forms. The answer to the question: Do you like music? Is Yes, I do. or No I don't. NOT Yes, I like. or No, I don't like.
do / does : emphatic use
We do not normally use do or does in affirmative sentences, Ali, but we can use them for emotive or contrastive emphasis when we feel strongly about something:
She thinks he doesn't love her, but he does love her. He really does!
You do look pretty in that new outfit! Quite stunning!
Are you all right? You do look a bit pale. Do please sit down.
I don't see very much of my old friends now, but I do still email them.
Was that a joke? I do believe you're teasing me!
When we are using the auxiliaries do and does for contrastive or emotive emphasis like this, we give them extra stress in pronunciation to make them sound louder, longer or higher in tone. When you see these words in print used in this way, they will normally be in italics or bold type or in CAPITAL LETTERS. Practise saying the sentences above with extra word stress on do and does.
The same rules apply when using did in the simple past tense, Hamid. It is normally used for making questions, in negative sentences and with shortened verb forms and can also be used for contrastive or emphatic use in affirmative sentences:
Did you go and visit your family last weekend? - Yes, I did.
Did you see everybody? - No, I didn't.
I saw my sisters, but my brother was away on business so I didn't see him.
Remember to give the auxiliary did extra stress in pronunciation in these examples of contrastive use:
Nearly every one was away on holiday, but I did manage to see Brenda.
I don't play very much sport now, but I did play a lot of tennis when I was younger.
I'm so worried at the moment that I don't' sleep well at night, but I did manage to sleep for six hours, last night.
ote that with modal auxiliary verbs, do and did are not used:
Can you play the clarinet? NOT: Do you can play the clarinet?
I can't play the clarinet very well. NOT: I don't can play the clarinet.
But I can play the recorder. NOT: I do can play the recorder.
forming questions using intonation
Note that we sometimes form questions by using a rising intonation at the end of the sentence. When we do this, we use the normal word order of affirmative sentences and don't use does and do in the simple present and did in the simple past.
We normally use this type of question when the speaker thinks he knows the answer to a question, but wants to make sure or when he wants to express surprise of disbelief:
You like hip-hop? But I thought you said you don't like any kind of rap?
You went to Brighton with Geoffrey? How could you? He's so boring!
I sold the diamonds. - You sold your mother's diamonds? How could you?