Translation:I bought bread and then watched a movie.
I was marked off for saying "the film" isntead of "a film"
Is this also correct... or how do you know the difference without articles?
"and then watched the film" would be "и только потом посмотрел(а) фильм" (and only after that watched the film). By inserting "only" you can shift the focus from the film and thus imply that it is the film mentioned ealier in the conversation
Can потом be translated as "afterwards"? This is how I've always known it.
"А потом" indicates the succession of events; in other words, when 'and then' means 'and afterwards', a more preferable Russian translation is "а потом". "И потом" is mostly used to introduce new information, it means "in addition to that", "also", "besides", e.g. Я не хочу ехать, и потом у меня нога болит = I don't want to go; besides, my leg hurts. In certain cases, you don't need 'потом' to translate "and then", e.g. "He may and then may not come" = Он может прийти, а может и не прийти.
The imperfective verb смотрела means either "was watching" or "have watched more than once". To describe a succestion of actions completed in the past we use perfective verbs.
I have translated it like: I had bought some bread and then I watched a movie. Is it incorrect?
"I bought the bread" (instead of "some bread" or just "bread") was not accepted. Why?
Report it. It should be accepted, because, strictly speaking, this meaning is possible, although less likely than "I bought some bread". Lots of Russian natives actually say "купить хлеб" when "купить хлеба" is meant - nobody cares. It's like saying 'a couple times' instead of 'a couple of times'.