Some recapitulation seems necessary as there have been more than one point advanced concerning the negative forms of adjectives.
First, when used predicatively, Japanese adjectives (let us call them thus, however tentatively) can assume two different negative forms.
The plain negative form is -ku (wa) nai.
この車は高く(は)ない。
kono kuruma-wa takaku(-wa) nai.
The polite negative form is -ku (wa) arimasen.
この車は高く(は)ありません。
kono kuruma-wa takaku(-wa) arimasen.
Both sentences mean, "This car is not expensive." The optional -wa particle is used to provide focus on the adjective.
When a sentence is joined to another with conjunctions (such as and, but), some caution is necessary.
In order to describe a pair of similar things, adjectives should assume -ku or -ishi ending.
Group I
Positive plain forms are;
姉は美しく、妹はかわいい。 ane-wa utsukushiku, imōto-wa kawaii.
姉は美しいし、妹はかわいい。 ane-wa utsukushiishi, imōto-wa kawaii.
The elder sister is beautiful and the younger sister is lovely.
Positive polite forms are;
姉は美しく、妹はかわいいです。 ane-wa utsukushiku, imōto-wa kawaii desu.
姉は美しいですし、妹はかわいいです。 ane-wa utsukushiidesushi, imōto-wa kawaii desu.
Please note that -ku cannot be followed by -desu, the politeness marker. The semantic difference between utsukushiku and utsukushiishi is that the former introduces a more-or-less objective parallelism but the latter emphasises the parallelism with an "as you might expect" attitude.
When the parallel is drawn between negated adjectives, the negative marker -nai assumes -ku or -ishi ending. The ending -ku being unable to be followed by -desu applies here too.
Negative forms are (the first two are plain forms, and the rest are polite forms);
Group II
姉は美しくなく、妹はかわいくない。 ane-wa utsukushikunaku, imōto-wa kawaikunai.
姉は美しくないし、妹はかわいくない。 ane-wa utsukushikunaishi, imōto-wa kawaikunai.
姉は美しくなく、妹はかわいくないです。 ane-wa utsukushikunaku, imōto-wa kawaikunai desu.
姉は美しくないですし、妹はかわいくないです。 ane-wa utsukushikunaidesushi, imōto-wa kawaikunai desu.
The elder sister is not beautiful and the younger sister is not lovely.
When a contrast betwee to things are described, particles -ga and -noni are applicable.
When the first clause contains an adjective in positive plain and polite forms;
Group III
姉は美しいが、妹は醜い。 ane-wa utsukushii-ga, imōto-wa minikui.
姉は美しいのに、妹は醜い。 ane-wa utsukushii-noni, imōto-wa minikui.
姉は美しいですが、妹は醜いです。 ane-wa utsukushii-desuga, imōto-wa minikui desu.
*姉は美しいですのに、______ 。 *ane-wa utsukushi-desunoni, . . . . .
The elder sister is beautiful but the younger sister is ugly.
The difference between -ga and -noni is similar to that between simple "but" and "although." The former is a more-or-less objective contrastive remark, whereas the latter is meant to emphasise the difference between the compared themes. Also note-worthy here is that -noni cannot be preceded by -desu.
The last group is contrastive remarks whose first clause contains an adjective in negative form. As with the group II, the negation adjective -nai takes the contrastive particles and the same rule applies as the group III;
Group IV
姉は美しくないが、妹はかわいい。 ane-wa utsukushikunai-ga, imōto-wa kawaii.
姉は美しくないのに、妹はかわいい。 ane-wa utsukushikunai-noni, imōto-wa kawaii.
姉は美しくないですが、妹はかわいいです。 ane-wa utsukushikunai-desuga, imōto-wa kawaii desu.
*姉は美しくないですのに、______ 。 *ane-wa utsukushikunai-desunoni, . . . . .
The elder sister is not beautiful but the younger sister is lovely.
Flaminius
