When conjugating adjectives, can we use the normal form of the adjective and use the copula です to indicate its form?

Question

This is specifically for い-adjectives, as な-adjectives work this way.
For example, when I want to say something is not delicious I'd usually say おいしくない.
Would it be possible to use おいしい じゃありません and would it mean the same thing?

Answer 1

It would mean the same thing, but wrong in terms of grammar.

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おいしくない:

• Question おいしくない？ (Delicious, isn't it?) valid
• Statement おいしくない (Not delicious) valid

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おいしいじゃありません:

• Question おいしいじゃありません？ (Delicious, isn't it?) valid
• Statement おいしいじゃありません Not valid

The question "おいしいじゃありません？" is a unique edge case, and you might never use it in your life.

The statement "おいしいじゃありません" looks/sounds wrong although the message is clear you are not enjoying that meal.

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Possible statements to deny おいしい:

• おいしくありません

Following works too:

• おいしいごはんではありません
• おいしいとは思いません

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Edit:

The above are basics. Hate to add complexity but there is yet another edge case where one is happy with the meal and say:

• Statement おいしいじゃありません (Isn't this delicious)
• Statement おいしいじゃありませんこと (Isn't this delicious)

This carries the same logic as the question, but is not a question. It's somewhere between a rhetorical question and a firm statement. These are advanced expressions which may be used with enough context and correct pronunciation.