eslHQ - What is ""S.+do+V1""?

Do in the simple present and past affermative is used for emphasis. When spoken it always heavily stressed. For example :

I do like your new hairstyle! = I really like your new hairstyle.
It was a great day, but I did feel tired by the end! = .... I felt really tired by the end.

It's used in the same way to contrast or deny a statement :

A : Will you be OK in Paris? I know you don't speak French...
B : I do speak French! I studied it for ten years.
A : So why didn't you speak to Yves in French when he was here.
B : We did speak in French. Every day.

This is no different from the way heavy stress is always put on the auxiliary verb for emphasis - but in all the other verb forms, the auxiliary is already present and therefore do doesn't need to be added.

A : I know you're not going to the concert tomorrow, so...
B : What? I am going to the concert!

A : David told me you won't be here tomorrow. Is that right?
B : No- why did he say that. I certainly will be here.