Uses of do
The auxiliary do is used:
1) to form the negative and interrogative of the simple present and simple
- He came. (Affirmative)
- He did not come. (Negative)
- Did he come? (Interrogative)
- He works. (Affirmative)
- He does not work. (Negative)
- Does he work? (Interrogative)
2) To avoid repetition of an ordinary verb, as in the following examples.
- ‘Do you know him?’ ‘Yes, I do.’ (= Yes, I know him.)
- ‘She sings well.’ ‘Yes, she does.’ (= Yes, she sings well.)
Do is also used to emphasize the affirmative nature of a statement.
- She did come.
- You do look upset.
Do can be used in the imperative mood to make a request or invitation sound
- Do be quiet.
Uses of Can
Can usually expresses ability.
- He can speak ten languages.
- I can knit.
- Can you lift this box?
Can is often used in the sense of may to give permission, though may is more correct.
- You can go now.
- You can take one of those books?
Now-a-days can is also being used to ask permission.
- Can I come in, sir?
Can is often used in negative and interrogative sentences to talk about possibility.
- Can this be true?
- No, it can’t be.
In affirmative clauses we use may to express possibility.
- It may rain.