い-adjectives

Japanese has two main categories of adjectives. Don't worry, they are not as complicated as the ones for verbs. It’s actually quite straightforward.

These two categories are い-adjectives (形容詞) and な-adjectives (形容動詞).

い-adjectives act similar to verbs and can conjugate. な-adjectives on the other hand act like nouns (and they often can also be used as nouns). We'll cover な-adjectives later.

Here's an example of an い-adjective in practice.

赤い火
Red fire

The reason why い-adjectives are similar to verbs is because they can conjugate.

To say some noun is/has some quality using い-adjectives, you can just make an “A is B” type of sentence with the adjective directly like this:

火が赤い
The fire is red.

You must not add だ after it:

*火が赤いだ

This is very wrong and not worth providing a translation.

However, attaching です is okay here, but it doesn’t have the role of a copula in this usage. It just adds politeness and doesn’t carry any meaning whatsoever. This is the first major way that だ and です are different. When it's not attached to a noun, です is just a politeness marker.

火が赤い
The fire is red

火が赤いです
The fire is red (polite)

Beyond です, There can also be some other constructions that put だ after an い-adjective, but they are specific set expressions. Don't be surprised if you see them, but don't worry about it now.

Like with verbs, you can also conjugate い-adjectives. You can replace い with かった or くない to attach a past or negative conjugation:

火が赤かった
The fire was red.

火が赤くない
The fire is not red.

The adjective いい is an exception and it becomes よい -> よかった / よくない.

Since the ない in verbs also behaves as an い-adjective, you can conjugate it like one and continue the chain of conjugations that makes it possible to build the long strings of verbs we mentioned earlier.

見る
to see (ichidan plain form)

見ない
to not see (negative)

見なかった
did not see (negative, past)

Adjectives can be turned into abstract nouns by replacing the い with さ. There are more ways to do this, but さ is by far the most common.

大きい
large/big (adj)

大きさ
size/largeness (noun)