-今天我们谈了数学，那当然了，我们还谈了物理========in this case, 那当然了=of course [This is right.In my opinion, 那当然=那当然了=当然=of course， for ever]

sorry, I was wrong...那当然=of course.

--今天我们谈了数学，那当然了，我们还谈了物理========in this case, 那当然了=besides

---你学习好吗？---那当然【of course】 -============ 那当然=当然=of course， we translated it like this!!

那当然 = conversational "obviously" or "duh", or "well, naturally..." is fine too. :)