"This movie was very interesting."

Translation: この映画はとても面白かった。

[Maximeius]

My answer:
このえいがはとてもおもしろかったです。

Is this not also correct?

[B_bethany]

Your answer is correct. です can be used after かった and it would be more polite as Rhiaaaaannon said. かった is showing that it is past tense.

When the です is missing it means the sentence is casual. We would speak that way with people we know well (friends & family), but not with your boss.

You could also say 「とても面白い映画でした」when you were talking if you wanted to use the past tense form of です. This sentence would work because the noun came before the でした.

[Moosey007]

この映画はとても興味深かった。’Interesting' is not necessarily ’おもしろい’

[Azraelios]

興味深い
That's a more advanced word that Duolingo doesn't teach yet. But I think it's definitely not wrong either. But neither is 面白い

[Rhiaaaaannon]

If you're talking about a story, you usually say おもしろい to mean interesting.

[circomatic]

Would you / s. o. mind adding Hiragana and Romaji , please, so this information can be of useful to more learners? Thanx!

[MarkWilliams123]

The below was marked as incorrect, but I don't see why? I know they're trying to teach the かった ending in this lesson but they're basically equivalent right?

この映画はとても面白いでした

[BananaBob13]

It seems like it should be right, but it's not. You need to handle the 2 kinds of adjectives differently, that's just Japanese grammar.

Just follow ayamedori's examples from the reverse thread:
"ayamedori:
In the case of い-adjectives you replace the い by かった:
面白い→面白かった
優しい→優しかった
With the exception of いい→よかった.

For な-adjectives, you just use だった/でした:
きれい→きれいだった
簡単→簡単だった"

https://forum.duolingo.com/comment/33196206

Also, you should refer to B_bethany's comment above.