I taught him history yesterday = 1) I taught him yesterday and am finished teaching him. He is not finished learning, though (Somewhat likely) 2) He has learned everything about history and I don't have to teach him any more. (much less likely) I was teaching him history yesterday = I was in the process of teaching him yesterday. He still has something to learn about history and I will probably be teaching him again. (Most likely event)

I taught him history yesterday = SIMPLE PAST TENSE I was teaching him history yesterday = PAST PROGRESSIVE (PAST CONTINUOUS) TENSE The PAST PROGRESSIVE is like the IMPERFECT tense in other languages such as French, Spanish and Italian. Use of Past Progressive puts emphasis on the course of an action in the past http://www.ego4u.com/en/cram-up/grammar/past-progressive: Example: He was playing football. two actions happening at the same time (in the past) Example: While she was preparing dinner, he was washing the dishes. action going on at a certain time in the past Example: When I was having breakfast, the phone suddenly rang.