off the subject
Introducing, addressing, or discussing things not relevant to or concerned with the subject at hand. Make sure you don't go off the subject during your lecture, or you'll just confuse your students. This is off the subject a bit, but what do you think about the recent economic trend in the Asian markets?
off the subject
not concerned with the subject being discussed. I got off the subject and forgot what I was supposed to be talking about. The speaker was off the subject, telling about his vacation in Hawaii.