The Difference Betweenよい (yoi) Vs いい (ii)｜Japanese Grammar (N5–N3)

Joy giving a thumbs up [Ii] vs. Joy conjugating grammar [Yoi])

“It is good.” (Ii desu) “It was good.” (Yokatta desu) Wait, where did “Ii” go? Why did it become “Yo”? Both よい (Yoi) and いい (Ii) mean “Good”. However, only one of them can change its form (conjugate). Let’s master the most important rule of Japanese adjectives with Joy and Ken!

🗣 Conversation Scene

Joy went to a new restaurant yesterday.

Joy

昨日（きのう）のレストランは、とてもいいかったです！料理（りょうり）もいいくないです。あ、あれ？ (Yesterday’s restaurant was very “iikatta”! The food was “iikunai”… Huh? Wait?)

Ken

Joyさん、ストップ！(笑) 「いい」は形（かたち）を変えられません。過去形（Past tense）は「よかった」ですよ。 (Joy, stop! haha. “Ii” cannot change its form. The past tense is “Yokatta”.)

💡 Explanation

The “Transformation” Rule.

1. いい (Ii) = Spoken / Present Only

Use for: Present Affirmative (It is good) “Ii” is the casual, spoken version of “Good”. You use it mostly in the dictionary form.

いいですね！ (That’s good!)
いい天気（てんき）。 (Good weather.)

2. よい (Yoi) = Grammar Base / Formal

Use for: Conjugation (Past, Negative, etc.) “Yoi” is the original form. When the word changes form (conjugates), you MUST switch to the “Yo-” base.

よくない (Yo-kunai) = Not good. (❌ Ii-kunai)
よかった (Yo-katta) = Was good. (❌ Ii-katta)
よければ (Yo-kereba) = If it is good. (❌ Ii-kereba)

📊 Conjugation Table

Form	Japanese	English
Present	いいです (*良いです)	It is good.
Negative	よくないです	It is not good.
Past	よかったです	It was good.
Te-Form	よくて	It is good, and…

🚧 Joy’s Mistake

(Joy talking about last week’s party) ❌ Wrong: パーティーは いいかった です。 (Grammar Error: “Ii” cannot add “-katta”.) ✅ Right: パーティーは よかった です。 (Correct: Switch to the “Yo-” base for past tense.)