It marked me wrong for using the kanji for 差しませんでした. I've reported it (Jan. 10, 2018).
This is wrong. "I did not use an umbrella" is "かさをつかいませんでした". This says "I did not raise an umbrella".
Before saying that this is wrong consider that the translation might be what is literally said in Japanese when saying that you used an umbrella. For example in Japanese you don't say that it's raining but that rain is falling/descending.
But there is another word for "use" which is 「つかう」 whereas in the raining example there is no other translation for it is raining
Maybe you don't use TSUKAU for "use" when talking about umbrellas, or some other objects, in Japanese?
For what it's worth, I really hate when the blocks of kana split out "masendeshita" like "mase - n - de - shita'
Meh. I don't mind it. It makes you work a bit harder for the win, is all.
Can でした be omitted?
Probably not. It's the polite past tense, without it you're saying "I DO NOT use an umbrella." Not quite the same meaning.