い-Adjective Conjugation: かったです vs でした

Question

When we first studied adjective conjugation in my Japanese class, I kept making the same mistake habitually; I would conjugate the past tense of い-adjectives with でした at the end instead of dropping the い and adding かったです. My teacher would correct me, and now I've more or less started doing かったです, but I still wonder about how "wrong" adjective-でした is.

○ あのテストは難しかったです。

×? あのテストは難しいでした。

Is the second sentence straight out wrong and/or extremely unnatural-sounding? Would anybody in Japan conjugate adjectives like this? Or does it possibly introduce a slightly different meaning?

Answer 1

あのテストは難しいでした to me sounds strange. If you search Google for テストは難しいでした, you get about 10 results, many of them written by foreign speakers. テストは難しかったです however gets far more results.

I wouldn't right out say that it's wrong to use 難しいでした as you can find many instances of it when you search for it. However, I think it's overwhelmingly used by females when you look through the results. It's just a speculation of mine, but it may sound slightly "cuter" or "younger" to use 難しいでした as in 確かに難しいでしたけれどーー字の雰囲気で何となく解りましたから――ー大丈夫でした and similar examples.

That said, I think people learning Japanese should use 難しかったです rather than 難しいでした.

Disclaimer: I'm not a native speaker, just my thoughts.

Answer 2

It is ungrammatical. No one will conjugate like that.