Chapter 1 of [Pretty Much Any] Japanese Textbook will teach you that いい (ii) means "good."

Then around Chapter 3 or 4, they'll tell you that the past tense is よかった (yokatta), meaning "was good:"

A: 映画はどうだった？
eiga wa dou datta?
How was the movie?
(Literally: "movie + は + how + was?")

B: よかった。
yokatta.
It was good.
(Literally: "was good.")

Well, they'd probably use ～でした (deshita) instead of ～だった (datta) and よかったです (yokatta desu) instead of just よかった (yokatta), because textbooks love stiff, polite Japanese.

Textbooks also treat people like idiots. (But that's a topic for another tirade.)

My point is: よかった is SO useful.

Here's why...

3 Ways Japanese People use よかった Every Single Day
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(I'll try to keep these short, sweet, and info-packed.)

#1 - Thank God!

You're having a friend's B-day party at your house, and your significant other is supposed to pick up the cake on his/her way home from work.

Your significant other was also wise enough to let his/her phone die, so you have no way of sending a reminder about picking up the cake!

The guests have started arriving.

You pray and pray and pray he/she won't forget the cake.

Then the door opens.

You ask:

A: ケーキ取ってきた？！
keeki totte kita?!
Did you get the cake?!
(Literally: "cake + take + come?")

(Note: The grammar form ～てくる could fill a whole lesson... and probably will some day.)

Since your significant other is not as stupid as you thought, apparently--is not a total detriment to humanity--he/she says:

B: うん。
un.
Yeah.

Now's your chance to say...

A: よかったー！
yokattaaaa!
Thank God! // Thank Heavens! // What a relief!
(Literally: "was good.")

This is Special Use #1.

Just say よかった (yokatta) and then drag out that last "ah" sound.

You'll probably see it written like よかった～ or よかったー.

I love one-word phrases.

#2 - Good thing I...

A lot of times in English, we use the phrase "Good thing I [verb-ed]" to express relief or gratitude that we did something.

I passed the test! (Good thing I studied!)

It's raining! (Good thing I bought an umbrella!)

You go to a party, and you see the most awesome, friendliest, giving person in the world who you only met one time but then lost his/her contact info! (Good thing I came to this party!)

In English, that's...

Good thing I + [past tense verb].

Good thing I studied!
Good thing I bought an umbrella!
Good thing I came to this party!

In Japanese, that's...

[Te-form of verb] + よかったー

Verbs:
to study = 勉強する (benkyou suru)
to buy = 買う (kau)
to come = 来る (kuru)

Te-forms:
する (suru) → して (shite)
買う (kau) → 買って (katte)
来る (kuru) → 来て (kite)

Good thing I...

勉強してよかったー
bennkyou shite yokattaaa
Good thing I studied. // I'm so glad I studied.
(Literally: "studied + was good.")

傘買ってよかったー
kasa katte yokattaaa
Good thing I bought an umbrella. // I'm so glad I bought an umbrella.
(Literally: "bought + was good.")

来てよかったー
kite yokattaaa
Good thing I came here. // I'm so glad I came.
(Literally: "come + was good.")

(Note: I left out "to the party" or anything like that, because in most situations, it wouldn't need to be included.)

Nuance Note: You don't actually have to draw out the end of your よかったー. You can just say よかった as you normally would, which lowers the amount of relief that you're expressing.

#3 - I wish I'd...

よかった is not only useful for expressing relief.

It's also my #1 word for expressing regret.

To express regret with よかった, you attach it to the Conditional ～ば (~ba) form of verbs.

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Conjugation Detour!
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If you're still a beginner, you probably don't know how to conjugate to the ～ば conditional form of verbs.

Here's a painfully truncated breakdown...

～ば Conditionals

(Note: "s.o." = "someone.")

Grade I Verbs
話す → 話せば
hanasu → hanaseba
to talk → if s.o. talks

飲む → 飲めば
nomu → nomeba
to drink → if s.o. drinks

Grade II Verbs
食べる → 食べれば
taberu → tabereba
to eat → if s.o. eats

寝る → 寝れば
neru → nereba
to sleep → if s.o. sleeps

Irregular Verbs
来る → 来れば
kuru → kureba
to come → if s.o. comes

する → すれば
suru → sureba
to do → if s.o. does

(So Many Notes: Japanese conditionals are pretty complicated, as there's a pretty wide variety of them. You'll get the hang of them over time.)

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End Detour!
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Like I was saying, if you snap よかった onto one of these ～ば words, you get:

I wish I'd [verb-ed].

勉強すればよかった。
benkyou sureba yokatta.
I wish I'd studied. // I should have studied.

傘買えばよかった。
kasa kaeba yokatta.
I wish I'd bought an umbrella. // I should have bought an umbrella.

行けばよかった。
ikeba yokatta.
I wish I'd gone (to the party). // I should have gone (to the party.)

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Conjugation Ninja Level-Up
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There's not really space to explain here, but you can also say, "I wish I hadn't [verb-ed]" by attaching よかった to the negative ～ば form.

So...

I wish I hadn't [verb-ed].

勉強しなければよかった。
benkyou shinakereba yokatta.
I wish I hadn't studied. // I shouldn't have studied.

傘買わなければよかった。
kasa kawanakereba yokatta.
I wish I hadn't bought an umbrella. // I shouldn't have bought an umbrella.

行かなければよかった。
ikanakereba yokatta.
I wish I hadn't gone (to the party). // I shouldn't have gone (to the party.)

That's some head-spinning verb conjugation!

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Exit Conjugation Abyss of Despair

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If you can master these verb conjugations, all you have to do is snap on よかった for super-native, super-high-level phrasing goodness.

First, you memorize the conjugation rules.

Second, you cry every time you forget them.

Third, you conjugate the verbs a million times.

Finally, you forget the conjugation rules and conjugate intuitively.

This final step exists--I promise you.

I went there once.

Then I came back to write this lesson.

Bonus Phrases

これ食べていい？
これ　たべて　いい？
Is it OK if I eat this?

最高のディナーでした。
さいこう　の　ディナー　でした。
That dinner was the best. // That was a fantastic dinner.

とてもよかったです。
It was really good.

買い物行ってくる。
かいもの　いって　くる。
I'm going to go (grocery) shopping. // I'm going to run to the store.
Literally: shopping + go and + come

話せばわかってくれるよ。
はなせば　わかって　くれるよ。
If you talk to her [him], she'll [he'll] understand.

飲めばいいじゃん。
のめば　いい　じゃん。
Why don't you just drink it?
Literally: if you drink (it) + good + right?

食べたいなら食べれば？
たべたい　なら　たべれば？
If you want to eat, then why don't you eat?
Literally: want to eat + if + if you eat?

寝れば治るはず。
ねれば　なおる　はず。
It should get better if you sleep. // I should be able to sleep it off.
Literally: if sleep + be healed + should

ミホも来ればよかったのに。
ミホ　も　くれば　よかった　のに。
I wish you [Miho] had come. // It would have been better if you [Miho] had come.

こうすれば出来るよ。
こう　すれば　できる　よ。
If you do it like this, you'll be able to do it.