【Difference】よい (yoi) vs いい (ii)

“It is good.” In Japanese, you say “いいです (Ii desu).”
“It was good.” In Japanese, you say “よかったです (Yokatta desu).”

Wait, why did “ii” change to “yo”?

This is because there are two words for “good”: いい (ii) and よい (yoi). They mean the same thing, but they have different grammar rules. This is a very important N5 grammar point.

This article will explain when to use いい (ii) and when to use よい (yoi).

Usage Explanation ①: いい (ii) – Spoken & Present Tense

いい (ii) is mainly used in conversation (spoken language). It sounds casual and natural.

However, there is one BIG rule: You can ONLY use “ii” in the present tense dictionary form. It cannot change its form (conjugate).

Example 1: (Present Affirmative)

それは、いいですね！(Sore wa, ii desu ne!)

That is good!

Example 2: (Modifying a Noun)

今日（きょう）は、いい天気（てんき）です。(Kyō wa, ii tenki desu.)

Today is good weather.

Example 3: (Casual Question)

これ、いい？(Kore, ii?)

Is this okay? (Casual)

Key Point: In daily speech, always say いい (ii) for “It is good.” But never try to conjugate it!

Usage Explanation ②: よい (yoi) – Written & Conjugation

よい (yoi) is the original form. It is mainly used in writing or formal speech.

Most importantly, when the word changes form (past tense, negative, etc.), you MUST use “yoi” as the base.

Past: よかった (yokatta)
Negative: よくない (yokunai)
Te-form: よくて (yokute)

Example 1: (Past Tense)

昨日（きのう）は、天気（てんき）がよかったです。(Kinō wa, tenki ga yokatta desu.)

The weather was good yesterday. (Not “ikatta”)

Example 2: (Negative)

気分（きぶん）がよくないです。(Kibun ga yokunai desu.)

I don’t feel good. (Not “ikunai”)

Example 3: (Formal Writing)

とてもよい経験（けいけん）でした。(Totemo yoi keiken deshita.)

It was a very good experience. (In an essay or report, “yoi” is better than “ii”.)

Key Point: Whenever you change the ending (conjugate), switch to the “Y” sound (yo-!).

Comparison: いい vs よい

Let’s check the conjugation table.

Form	いい (ii)	よい (yoi)	Correct Usage
Present (Dictionary)	いい (ii)	よい (yoi)	Use いい for speaking. Use よい for writing.
Negative (Not good)	いくない (ikunai)	よくない (yokunai)	Always use よくない.
Past (Was good)	いかった (ikatta)	よかった (yokatta)	Always use よかった.
Te-form (Good and…)	いって (itte)	よくて (yokute)	Always use よくて.
Adverb (Well)	いく (iku)	よく (yoku)	Always use よく.

Common Mistake Examples

Mistake 1: “It was good!”

Wrong:: 映画（えいが）は、とてもいかったです。(Eiga wa, totemo ikatta desu.)
Correct:: 映画（えいが）は、とてもよかったです。(Eiga wa, totemo yokatta desu.)
Why?: “Ii” does not have a past tense form. You must use the base “yo” + “katta”.

Mistake 2: “Not good”

Wrong:: それはいくないです。(Sore wa ikunai desu.)
Correct:: それはよくないです。(Sore wa yokunai desu.)
Why?: “Iku” means “to go”! For “not good,” use “yoku” + “nai”.

Practice Quiz

Let’s check your understanding! Choose the correct word.

Q1.

（話（はな）している時）「それ、( ) アイデアですね！」

(Speaking) “That is a ( ) idea!”

よい (yoi)
いい (ii)

Q2.

昨日のテストは、あまり ( )。

Yesterday’s test was not very ( ). (Past negative)

いくなかったです (ikunakatta desu)
よくなかったです (yokunakatta desu)

Q3.

頭（あたま）が ( ) なりたいです。

I want to become smart (head becomes good).

いく (iku)
よく (yoku)

Quiz Answers

Q1. (b) いい (ii) – For spoken, present tense, “ii” is the most natural.

Q2. (b) よくなかったです – Conjugation (Negative + Past) always uses the “yoi” base.

Q3. (b) よく – Adverb form (“becomes good”) always uses the “yoi” base.

Summary: The Golden Rule

It is simple to remember:

Speaking (Present tense): Use いい (ii).
Example: いいです (It is good).
Changing form (Past/Negative): Switch to よ (yo).
Example: よかった (It was good), よくない (Not good).

Remember: NO “ikatta”! YES “yokatta”!

Frequently Asked Questions (FAQ)

Q1: Can I say “ikatta” (it was good)?: A1: No, you cannot. いい (ii) cannot change its form. When you want to say the past tense, you must switch to the base よい (yoi). So, “yoi” + “katta” = よかった (yokatta).
Q2: Which one should I use when speaking?: A2: In daily conversation (speaking), please use いい (ii) for the present tense (e.g., いいですね). It sounds friendly and natural. よい (yoi) sounds a bit formal or stiff in casual speech.
Q3: What about “kakkoii” (cool)? Does it follow the same rule?: A3: Yes! “Kakkoii” comes from “kakko” (appearance) + “ii” (good). So, the past tense is かっこよかった (kakkoyokatta), not “kakkoikatta”.