Introduction to Japanese Adjectives
To study this grammar point, please make sure you understand the following:
Adjectives are used to modify nouns. There are 2 types of adjectives, namely な-adjective (na-adjective) and い-adjective (i-adjective), in Japanese grammar.
You need to be able to distinguish between な-adjectives and い-adjectives because they have different rules for their modified forms.
By attaching な to the end of some noun, it becomes a な-adjective.
When you look up the dictionary, for instance, 元気 (/genki/ health) is both a noun (名) and an adjectival verb (形動). That means it is a な-adjective, and you can use it to modify another noun.
The stem of a な-adjective is a noun (without the suffix な). This is the base for all other modified forms of the adjectives.
Examples of な-adjectives:
An い-adjective always ends with a hiragana suffix い. It is specified as adjective (形) in a dictionary.
The stem of an い-adjective is the adjective without the suffix い. This is the base for all other modified forms.
Examples of い-adjectives:
One major difference between い- and な-adjectives is that, the stems of な-adjectives act like nouns, but い-adjectives do not have such a property.
As a result, for many modified forms, だ needs to be attached to な-adjective, but never to い-adjective.
Never attach だ to い-adjectives.
Notice that some な-adjective, for example, きれい (/kirei/ clean), looks like an い-adjective. But if you look up the word in kanji, it is writen as 綺麗, and it does not have an い suffix. So it cannot be an い-adjective.
On the other hand, some adjective stems can serve as both い- and な-adjectives. For example, 大きい and 大きな are both common adjectives in Japanese.