Let's talk about i-adjectives.
Most textbooks will tell you that there are 2 kinds of adjectives: i-adjectives and na-adjectives.
But is this really true?
I-adjectives behave like verbs. Na-adjectives behave like nouns.
You can conjugate i-adjectives. They don't need verbs, because they act like verbs.
重い (omoi) is heavy.
重かった (omokatta) was heavy
重くない (omokunai) is not heavy
重くなかった (omokunakatta) was not heavy
Because i-adjectives already behave like verbs, it is actually wrong to use verbs with them!
Believe it or not, the following sentences are actually wrong:
重いです。 (omoi desu)
重かったです。 (omokatta desu)
How it that possible? This is a serious problem that apparently grammarians have not solved yet.
The construction [i-adjective + です] was created as an alternative to ございます (gozaimasu).
Strictly speaking, the only way to create the polite form of i-adjectives is by attaching ございます, which is now considered archaic.
plain form: 重い (omoi)
polite form:重うございます (omōgozaimasu)
But because ございます is now considered archaic, they had to come up with an alternative.
So at the beginning of the 20th century people started to use ですat the end of i-adjectives, even though it was reviled by grammar books.
According to the site below in 1952 the Japanese government published a report addressing this problem and advocating for a simplification of the polite form and accepting the structure [i-adjective + です] which was already widely used in spoken language.
The problem was aggravated by textbooks for foreigners which teach the polite form first instead of the plain form (which is the way Japanese people normally learn it).
So most textbooks will have sentences like:
この家 は 大きい です。(kono ie wa ookii desu)
This house is big.
even though it is grammatically wrong!
If です were to be considered correct, so all variations of ですshould also be correct.
But most Japanese people will cringe if you say any of the following sentences:
It doesn't make sense because だ、です、である are all the same word and should be interchangeable.
Why is only です considered correct if all its other variations are wrong?
Because strictly speaking です is also grammatically wrong and should not be used with i-adjectives.
です is only TOLERATED because there is no other way to make the polite form for i-adjectives.
I think it is very interesting that textbooks will actually teach you wrong grammar as the lesser of two evils.
And it is also interesting that people want to be polite at all costs, even at the expense of grammar!