Emphasis

Do for contrastive emphasis

Do can be used with a positive verb for emphasis. Do is used in this way when we want to correct or contradict information.

• I know you're not going out, but if you do go out, dress up warmly.

Do is also used in this way when we want to imagine the consequences of an impossible or very unlikely situation.

• If the asteroid did collide with Earth it would do so at more than 17,000 mph.

Do is not necessary when there is already an auxiliary verb.

• If the asteroid was on a collision course with Earth, a way to alter its orbit would have to be found.
• He's not my type of person, but you're right, he is very charming.

To emphasise an auxiliary verb, decontract it and stress it.

• "He's not very charming."
  "On the contrary, I think he is very charming."