How to Conjugate Japanese Adjectives: い-Adjectives, な-Adjectives, and Irregular Ones

Did you know Japanese adjectives conjugate? In English, "happy" stays "happy" whether you're talking about yesterday or tomorrow. But in Japanese, adjectives change their form based on tense, politeness, and whether you're making a positive or negative statement. I'm going to walk you through exactly how to conjugate both types of Japanese adjectives, including the irregular forms that always trip people up. By the end, as a Japanese learner, you'll be able to describe things in different tenses and connect multiple adjectives together.

The two types of Japanese adjectives

Before we get into conjugation, you need to understand that Japanese has two completely different adjective categories. This matters because they conjugate differently.

You might be wondering: if な from a na-adjective is removed, it looks like an i-adjective, right? Nope. The key difference is that na-adjectives don't have that い ending as part of their core form. The い in kirei isn't the same grammatical element as the い in atsui. Na-adjectives behave more like nouns in how they conjugate.

I-adjective conjugations: The complete system

I-adjectives are pretty straightforward once you learn the pattern. The い at the end is what changes for different conjugations.

Present affirmative (Dictionary form)

This is the basic form you'll find in dictionaries. Just use the adjective as-is:

In a sentence:
。
Today is hot.

Present negative

Drop the final い and add kunai (くない):

In a sentence:
。
Today is not hot.

Past affirmative

Drop the final い and add katta (かった):

In a sentence:
。
Yesterday was hot.

Past negative

This one combines both changes. Drop the い, add kunakatta (くなかった):

In a sentence:
。
Yesterday was not hot.

Te-form for i-adjectives

The te-form lets you connect adjectives or create a reason. Drop the い and add kute (くて):

Example:
。
This restaurant is cheap and delicious.

Ku-form (Adverbial use)

Want to turn an adjective into an adverb? Drop the い and add ku (く):

Example:
。
To run quickly.

The irregular adjective: Ii (いい)

Here's where things get annoying. The adjective ii (いい) meaning "good" is irregular. When you conjugate it, you actually use forms based on yoi (), the older version of the same word.

You'll hear people say ii in conversation, but the conjugations come from yoi. Just memorize this one separately because it doesn't follow the standard pattern.

Na-adjective conjugations: Working with desu

Na-adjectives behave differently because they work more like nouns. In polite speech, they use desu (です), and the conjugation happens through the copula, not the adjective itself.

Present affirmative

Add desu (です) after the adjective:

In a sentence:
。
This room is clean.

Present negative

Add dewa arimasen (ではありません) or the casual ja nai (じゃない):

Past affirmative

Add deshita (でした):

Past negative

Add dewa arimasen deshita (ではありませんでした) or ja nakatta (じゃなかった):

Te-form for na-adjectives

Add de (で) after the adjective:

Example:
。
This town is quiet and pretty.

Tips for learning Japanese adjective conjugation

Here's what actually helped me get comfortable with adjective conjugation:

Why adjective conjugation matters for Japanese learners

Learning to conjugate adjectives opens up way more expression in Japanese. You can talk about past experiences, describe what something wasn't like, and connect multiple descriptions smoothly.

The conjugation system also helps you understand Japanese grammar structure better. Once you see how adjectives, verbs, and the copula all work together, the whole language starts making more sense.

Anyway, if you want to practice these conjugations with real Japanese content, Migaku's browser extension and app let you look up words and grammar patterns instantly while watching shows or reading articles. You can see adjectives in their conjugated forms and get instant explanations. There's a 10-day free trial if you want to check it out.

FAQs

Tackle the difficult part to improve your Japanese from the very start!

For any learner serious about Japanese, adjective conjugation is difficult, but also fundamental, just like verb conjugation. You'll use these forms in basically every conversation. The advice is to start early and learn the rules. Memorize basic patterns and high-frequency vocab from the very start. Then, as you invest more and more time in media consumption and course learning, your brain will gradually internalize the rules you've learned.

Pick up the rules, and leave the rest to time!