いい

いい
adjective (i-adjective)
good, fine, nice
1. good, fine, nice
A general positive evaluation. Can describe quality, condition, character, or suitability. One of the most frequently used adjectives in Japanese.
いい天気(てんき)だ。
It's nice weather.
この(ほん)いいです。
This book is good.
He's a nice person, isn't he?
This restaurant has good food and the staff are friendly.
This town is a nice, quiet place. It's easy to live in and convenient for shopping.
2. okay, all right (permission/acceptance)
Used to give permission or indicate that something is acceptable. いいですよ (That's fine / Go ahead).
いいです
That's fine. / Go ahead.
May I sit here?
May I use this?
Is tomorrow okay?
3. no thank you, I'm fine
Used to politely decline an offer. When someone offers you something, いいです can mean 'No, thank you' or 'I'm fine (without it).'
もういいです。
I'm fine now. / That's enough.
コーヒー?—いいです。
Coffee? —No thanks, I'm fine.
Shall I help? —No, I'm fine.
レシートいいです。—はい、いいです。
Do you need a receipt? —No, I'm fine.
Would you like another glass of water? —No thank you, I've had enough. Thank you.

いい has irregular conjugation. The negative, past, and other forms use the stem よ- from the alternate form ()い: よくない (not good), よかった (was good), よく (well). The form いい is used only in the dictionary/plain present form. ()い is more formal/literary. In the pattern 〜てもいい (may I...?), いい grants permission.