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Question 去年の成績がよかったです

Zizka

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I've had this question which kept coming back and it might be stupid but:
How come we don't write:
去年の成績がよかったでした
Could I have written でした instead of です? If so, would it change the meaning?
 
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Only the title sentence is correct in standard Japanese.
The reason it is done this way is thought to be because です is not the copula here, it is only added for politeness.

Notice that you cannot say ✖成績がいいだ ✖成績がよかっただ(った) ; if the copula does not follow い形容詞 in plain form, then what is です doing? Simply adding politeness to an already complete sentence.

去年の成績がよかったです
去年の成績がよかったでした
 

Zizka

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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.
 
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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.
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).

これはりんご
これはりんごです
朝ごはんはりんごだった
朝ごはんはりんごでした


There is no copula in 「成績がよかった」. It is ungrammatical to put the copula after an i-adjective ; for all intents and purposes i-adjectives are a type of verb, so you cannot say ✖ いいだ anymore than you can say ✖ するだ.

That makes putting です after よかった seem strange, and that strangeness is sometimes explained by considering です to be only for politeness.

I wonder what textbook you are using, because there is usually a section early on to cover this and several other questions you've asked.
 

Zizka

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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
 

Toritoribe

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As Chris-san wrote, です after an i-adjective is not copula but a polite suffix. In fact, this です doesn't conjugate.
e.g.
present affirmative: いいです
present negative: よくないです
past affirmative: よかったです
past negative: よくなかったです

Note that this is applied only to です after an i-adjective. です after a na-adjective does conjugate.

present affirmative: この部屋は静かです。
present negative: この部屋は静かでは/じゃありません。
past affirmative: この部屋は静かでした。
past negative: この部屋は静かでは/じゃありませんでした。

You must know that there is anther polite negative form ~では/じゃないです for na-adjectives. This is a bit confusing, but this です after ない doesn't conjugate since ない performs as an i-adjective.

present negative: この部屋は静かでは/じゃないです。
past negative: この部屋は静かでは/じゃなかったです。
 

Zizka

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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
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.

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.
That was my first impression until I was told otherwise.
 

Toritoribe

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Well, actually, this です is usually not classified into copula in Japanese grammar, so "As a polite suffix it works but in that case it doesn't play the role of copula." is hard to say "accurate". It's a polite suffix, not copula that works as a polite suffix.

EDIT:
That was my first impression until I was told otherwise.
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."
 

Zizka

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Well, I’m under the impression I wasn’t right judging from the explanations give it doesn’t have anything to do with よさたった being in the past. The reason why です is at the end of the sentence I provided is just a polite suffix. That’s what I took away from it.
 

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