Adjectives made simple

The adjectives in Japanese are not so simple. They are divided to two categories and are conjugated between tenses. But they follow strict rules and there is only one irregular adjective, so when you will get a hang on this, it will be simple. Let's start.

Two types of adjectives are い- adjectives and な-adjectives and the irregular adjective is いい (good).
The interesting thing I noticed that the な-adjectives are behaving like nouns while conjugated (Yes, in Japanese the nouns also got infected with this).

Now to go over the rules, lets pick two adjectives. Let's take  きれい (な) (beautiful) for な adjective and ちいさい (small) for い adjective.

To modify a noun correctly with adjectives you need to put them before the noun:

きれいな ねこ です. A beautiful cat.
ちいさい ねこ です. A small cat.

 Or after the noun:

この ねこ は きれい です. This cat is beautiful. (see the な disappeared)
この ねこ は ちいさい です. This cat is small.

So far so good. But there is a reason the な is written in brackets. When conjugating, it tends to disappear a lot. 

な-adjectives

As previously said, the な adjectives behave like nouns:

present affirmative: きれいです. present negative:  きれいではありません.
past affirmative: きれいでした. past negative: きれいではありませんでした. 

Affirmative  ~です and ~でした can be replaced with less polite:
~だ and ~だった

Negative ~ではありません and ~ではありませんでした can be replaced with: 
~じゃありません and ~じゃありませんでした- less polite 
~じゃない and ~ではなかった- least polite

い- adjectives

They have a different but predictable pattern:

Present affirmative: ちいさいです. present negative: ちいさくありません.
Past affirmative: ちいさかったです. past negative: ちいさくありませんでした.

In less polite form:

Affirmative just removing the ~です.

Negative  ~ありません and ~ありませんでした replaced with: ~ない and ~なかった respectfully.

Irregular いい

Present affirmative: いいです. present negative:  よくありません.
Past affirmative: よかったです. past negative: よくありませんでした.

And less polite form present よくない and past よくなかった.

And now for additional usage of adjectives:

Joining sentences with て form 

For longer sentences you can use the て form (yes it's not exclusively for verbs):

この ねこ は きれい で, ちいさいです. - This cat is beautiful and small.
この ねこ は ちいさくて, きれいです. - This cat is small and beautiful.
この ねこ は よくて, きれいです. - This cat is good and beautiful.

Noun replacment

When using the same noun and different adjectives, you can replace it with の or (ひと for people).

くろい この が あります. しろい の も が あります