"It is good."
Sorry, the adjective form is usually pronounced いい in modern daily speech. You can almost just forget よい exists for now. ^^
The reason I put よい is because that's how it can also be pronounced, especially in formal speech. In certain phrases and expressions it is always よい.
When this adjective is written in kanji, it is usually written 良い. This how it is written for both the よい and いい pronunciations.
However, いい isn't used for any other conjugations of this adjective. よい is the form used in all the other conjugations such as for negative, past tense, adverb form,...
- いい (or よい)【良い】"is good" (non-past)
- よかった【良かった】"was good" (past)
- よくない【良くない】"is not good" (negative)
- よくなかった【良くなかった】"was not good" (negative past)
- よく【良く】"well" (adverbial)
- よくて【良くて】"good, ..." (conjunctive)
- よければ【良ければ】"if good" (conditional)
More or less like that anyway. ^^
Here's something to get your head around... "How's the coffee?" いいです。It's good. "Would you like some coffee?" いいです。No, thanks. That has caught me out so many times when offered something. The second いいです is more like, I'm happy with my situation. It's a bit like saying "I'm good." Remember that and you'll save yourself from having to run after the server/waiter for a refill (^ ^)/