“The teacher has explained a new matter to us”
You must first to indentify both the ‘receiver’ and ‘doer’of the action in your initial active sentence. “Us’ is the receiver (indirect object) and the subject ‘teacher’ is the doer.
Then, to form a passive, you have to invert the sentence by shifting the receiver ‘we’ to the subject position in your passive sentence.
Hence, “ We have been explained a new matter by the teacher”
“We” now becomes the subject of your passive sentence and the ‘teacher’ is a prepositional object.
Something like “they”(professor and his coed ?) get into a horny sixty-nine and “change” and so on and so forth, hahaha :))