Do NOT call this series Onegai Sensei If it were called Onegai Sensei it would be titled But it isn t it is titled note the katakana reads tei cha The katakana is derived from the English word teacher thus it is known as Onegai Teacher Those who do call it Onegai Sensei have literally took the English word teacher and then translated back into Japanese and romanized it once again into sensei which if you think about it is just their way to show off that they know a little Japanese