[Matterwave]

Hey, I have a question about speaking in past tense. I'm currently using Rosetta stone, so some of the subtleties may not have occurred to me.

If I say a sentence like:

"Mae wa, watashi wa sakkaa o shite imashita."

Could I instead say:

"Mae wa, watashi wa sakkaa o shimashita."

Is there a difference?

Basically, I'm asking what is the difference between something like "hashirimashita" and "hashitte imashita".