Looking for quick grammar explanation on this

I’m doing the VLJ course for the JET program and I’m wondering about this question:

image

I thought it was (2), but was marked incorrect, (1) is the correct answer. Is there any particular rule that I’m missing? Would 面白いだけど also be acceptable or is it just plain wrong for some reason? Thanks in advance :smiley:

Nouns and な-adjectives require だ before けど. Verbs and い-adjectives cannot have だ before けど. Since むずかしい is an い-adjective, it cannot have だ. That’s all.

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Ahh I see now! Perfect! Thanks so much :smiley:

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Note that in the common “i-adjective + です” construct, the です is there for politeness only (to fill a void due to the lack a -ます equivalent for i-adjectives), it doesn’t have a grammatical role.
In particular it doesn’t conjugate (the past polite is -かったです and not *-いでした for example).

And when the use of polite ending form is not possible (in relative sentences, or in more casual speech) that です doesn’t become だ.

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Why is it not

日本語はむずかしいですけど、おもしろいです。

It can be! But that’s not one of the selections.

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