I often hear these mistakes made by German speakers who are learning English.
They say: The conference has begun at 11 yesterday.
The wrong tense has been used here. Remember the word yesterday indicates finished time. Therefore use the past simple. For unfinished time use the present perfect (have / has + 3rd form of the verb.)
Correction: The conference began at 11 yesterday.
Some further examples:
I worked a lot last week (finished time; last week is over, done, finished.)
I have worked a lot this week (unfinished time; this week is not finished, it is ongoing.)
She drank three coffees yesterday (finished time; yesterday is over, done, finished.)
She has drunk three coffees today (unfinished time; today is not finished, it is ongoing.)
Have fun learning English!
Louise