auxiliary verbs - When to use "do" in phrases which are not questions?

Example: You do need to drink more water.

or

You need to drink more water.

What is the difference?

Answer

Using do there adds emphasis and affirmation. Like if I told you

Me: You need to drink more water

You: I drink enough water

you go to the ER for dehydration

Me: You do need to drink more water!

I'm emphasizing that I was correct and you weren't getting enough water. Or if someone says

1: You didn't read that book.

2: I did read the book

you're emphasizing that you in fact did read that book; it's stronger than

I read the book

Addition: It's also used to ask questions, in verb tenses that don't use an auxiliary verb:

Do you exercise every morning?
Did you drive to the party?

But:

Have you exercised this morning?
Are you driving to the party?
Were you exercising this morning at about 6 o'clock?