Japanese has a fairly complex grammar, but it's grammatical rules are straightforward. As such Japanese adjectives can be easily classified. Classification of Japanese adjectives becomes very important in the learning process. I would say this could be the next step after the hiragana and katakana alphabets. Conjugation rules of a given adjective are dependent on the group it belongs to.
Japanese adjectives can be classified into two big groups
Japanese adjectives can be classified into two groups. Exceptions as always exist, namely when an adjectives looks like it belongs to a group but eventually it belongs to another one. The group, or strictly speaking the type of the Japanese adjective can be easily identified by considering only the ending of the adjective. There are adjectives that are ending in "i" (い hiragana from now on) and there are adjectives that are ending in "na" ( な hiragana from now on ). Consequently い (in Japanese: い形容詞、いけいようし) and な (in Japanese: な形容詞、なけいようし) type adjectives can be differentiated.
When you are learning Japanese, it is very important that you accurately master the grammar
Adjectives that even though have い endings but they are な-types are exceptions.
In addition to the above rules, adjectives that are introduced from foreign languages become な-type adjectives. （in Japanese: 外来語、がいらいご）
I would like to mention something here, that even most of the native speakers don't realize. So if we take the first example, handsome means handsome, no problem. But. If we say "smart" in English, it does mean something like "clever", however in Japanese it is used as "slim". As a result, most of the Japanese teenagers think that smartphone is actually "slim-phone". The last one in the list, which is a transformed version of the English "naive" means innocent or sensitive, which is again a misuse.
Japanese words have two forms. One is called the "dictionary form (Japanese: 辞書形、じしょけい）" and the other one is called the conjugated form. These are simple definitions, namely that the word that we can find in the dictionary is in "dictionary form" and if it has been conjugated, then it is in conjugated form.
Let's see a basic example:
い-type adjective and it's conjugated form:
な-type adjective and it's conjugated form:
When you would like to create an adnominal form you just have to simply put the adjective before the noun.
Using the previous example:
Adnominal form of the い-type adjective:
Adnominal form of the な-type adjective:
Alright, let's see two common Japanese adjectives. One い-type （high, expensive: 高い、たかい） and one な-type （bustling: 賑やかな、にぎやかな）adjective. Their conjugation is as follows:
Some of the forms have different variants. For instance you can use 賑やかじゃない instead of 賑やかではない. I tried to collect the most common conjugated forms.
Learning the conjugation of Japanese adjectives takes a longer time, but it is far from impossible
WARNING! The sentences above lack of any form of politeness. As such never use them in their current form when talking to your superior (boss, teacher, professor, stranger). Go for it, if you are talking to your girlfriend, boyfriend! If you would like to make the sentences polite, I recommend you to read my article related to "keigo".
Source 1: http://kyoan.u-biq.org/na-adjective.html
Source 2: http://kyoan.u-biq.org/i-adjective.html