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What? No. The copula is a word that isn't a verb, but means one thing 'is' the other thing. (We don't have it in English, we just use 'to be' for this).So if I understand you correctly, if a copula is already present, there's no more need for です and so it's therefore simply added for politeness. In that case, it is not conjugated regardless of the context.
Could I have written でした instead of です?
If so, would it change the meaning?
So what I've said is above is accurate. As a polite suffix it works but in that case it doesn't play the role of copula.So it's ungrammatical to put a copula after an i-adjective except when it expresses politeness as with:
去年の成績がよかったです
That was my first impression until I was told otherwise.The term よかった already contains the idea of past tense. The term でした also contains the idea of past tense. Using such a double-negative would be a mistake.
However, that way of thinking "the tense is provided by just one of the two; whether いい/よかった or です/でした" can't explain why いいでした or よくないでした are ungrammatical. If the past tense can be provided by whether よかった or でした, いいでした must be also acceptable. Thus, it's not "でした is not necessary since よかった already provides the past tense" but "です after i-adjectives is not a copula but a not-conjugated polite suffix, which can't provide the tense."That was my first impression until I was told otherwise.
That was my first impression until I was told otherwise.