In the English language, As you may already know, the auxiliary verb "do" is used in the Present Simple Tense only in the interrogative (questions) and negative but not in the positive.
For example, "Do you speak English. Yes, I speak English. No, I do not speak English."
However, when we wish to be emphatic about something, we can also use it in the positive, and say "Yes, I do speak English!"A In such cases, we put a heavy accent on the word "do".
We usually use the emphatic "do" when someone denies something we say and which we know to be true.
For example, we might say "Mr. and Mrs. Brown go to the cinema every evening", and then someone might deny it and say "No, they don't go to the cinema every evening", and we then say emphatically "Yes, they do go to the cinema every evening".
Another use of the emphatic "do" is in an exclamation.
For example, "I do like this book! or He does speak English well!"
Of course, the emphatic form can also be used in the Past Tense.
For example, "I did speak English well ten years ago" and "I did like that book I read last week."