Emphatic do

Sometimes we add "do" before the main verb of the sentence.

This is called the Emphatic do, and it is very common in spoken English when we want to add emphasis to affirmative sentences.

"I did follow the instructions to the letter.” This example suggests there was a problem and the speaker emphasises that they did everything they were supposed to do. 

With really or absolutely, the Emphatic "do" is also common.

“I really did follow every instruction to the letter.”

For questions or confirmation :
“They do understand the gravity of missing this deadline, don’t they?”