"오늘은 연습이 어려웠어요."
Translation:Practice was difficult today.
"Today practice was difficult" is an incorrect sentence. "Today, practice was difficult" or "Today's practice was difficult" would be better.
Yeah, that's probably because of Duolingo's decision to not pay attention to capitalization and punctuation.
Hence, the ones without are also included in their database and can show up in the exercises.
Yeah but I would guess, speaking from my own experience, that people talk like that
the correct answer is still marked wrong since identified above 2 months ago.