Describing things with い- and な-words
Say この本はおもしろいです. Then change only the description to おもしろくないです. This one pair is enough to begin; な-adjectives and exceptions come later in the lesson.
1. An い-adjective at the end of the sentence
In この本は新しいです, the described object comes before は. 新しい is the property and comes directly before です. The final い is already part of the Japanese base form. です makes the statement polite but does not change the adjective's meaning.
For the negative, replace the final い with くない and add です in this lesson pattern: 新しい → 新しくないです. Do not keep both: 新しいくないです is wrong. With an ordinary い-adjective, do not automatically use じゃありません either; in this lesson that pattern belongs to な-adjectives.
この本は新しいです。
この本は新しくないです。
2. Recognise words before changing them
Practise い-adjectives in pairs: 新しい／古い, おいしい／おいしくない, and おもしろい／おもしろくない. 安い means cheap; 高い, which returns later, can mean expensive or high depending on what you describe. Pair each word with a concrete object or dish before changing its form.
The な-adjectives in this lesson are 便利, 静か, きれい, 暇, and 有名. The label ‘な-adjective’ refers to behaviour you will see before a noun in lesson 18. At the end of a positive sentence, there is no な here: 便利です and 静かです. Remember each word together with its class.
3. A な-adjective at the end of the sentence
In この部屋は静かです, 静か comes directly before です. Do not insert な there: 静かなです is not the pattern for a sentence-final predicate. For the negative, replace です with じゃありません: 静かじゃありません. The same change works here with 便利, 暇, and 有名.
きれい visibly ends in い, but belongs to this な-group. Its final sound is part of the whole word and is not the inflecting い of 新しい. Therefore use きれいです and きれいじゃありません. A form such as きれいくないです follows the wrong word class.
この部屋は静かです。
この部屋はきれいじゃありません。
4. The fixed exception pair いい／よくない
いい, ‘good’, belongs to the い-adjectives, but its negative is irregular: いいです becomes よくないです. Learn this as a fixed pair. Do not form いいくないです or いいじゃありません when you mean the standard form taught here.
This lesson practises only the present positive and present negative. Past forms of いい come later. During recall, compare three types: 新しい → 新しくない, きれい → きれいじゃありません, and いい → よくない. This keeps the exceptional pair clearly separate.
5. Classify before producing
Choose four words: 新しい, おいしい, 静か, and きれい. First identify each word's group. Then make a positive and negative sentence about a fictional book, dish, or room. Check the form only after making your own choice.
The final check covers classification and exception recognition, followed by a regular い-negative and the fixed いい pair. In the open application you also compare な-adjectives, but unrestricted descriptions receive only criteria and self-reflection.