The present or the past of a verb in the active voice may be expressed with emphasis by means of a verb-phrase consisting of DO or DID and the infinitive without TO.
Such a phrase is called the emphatic form of the present or past tense.
“I do see you" and “I did go" differ from “I see you" and “I went" merely in emphasis. Hence do see is called the emphatic form of the present tense of see and did go the emphatic form of the past tense of go.
In questions and in negative statements the emphatic forms are used without the effect of emphasis.
Did you go? I did not go.
Note : DO often stands for some other verb which has just been used such as….
1. Jack swims better than I do.
2. You looked as tired as she did.
This idiom comes from the omission of the infinitive in the verb-phrase such as…..
1. Jack swims better than I do [swim].
In poetry and older English the verb-phrase with do or did in declarative sentences often carries no emphasis, but merely takes the place of the present or past such as….