I watched his presentation on wednesday
Both are correct translation but use of 'did' emphasizes more action of speaker, like in same way as 'ही' is used in Hindi to put more emphasis on a word. These two sentences don't mean exactly same thing.
"I watched his presentation on Wednesday" is also an accepted translation.
Yaa it is correct.... Did always use either interogative or negtive... As a indefinite we cant use did
Simple tense me did ka use kse ho skta h