На русский язык обе эти фразы переводятся как "я был(а)", но употребляются в разных случаях. I was употребляется, когда мы хотим сказать о каком-то определённом промежутке времени в прошлом. Например: I was in Canada last winter — я был в Канаде прошлой зимой...

aren't = are not can't = cannot didn't = did not doesn't = does not don't = do not haven't = have not hasn't = has not he'd = he had/ should/ would he'll = he will he's = he is/ has here's = here is/ has I'd = I had/ should/ would I'll = I shall/ will I'm = I am I've = I have isn't = is not it'll = it will it's = it is/ has mayn't = may not mustn't = must not shan't = shall not she'd = she had/ should/ would she'll = she will she's = she is/ has shouldn't = should not that'll = that will...