I love i-adjectives. And I think you should too.

What's an i-adjective? asks student who never reads these lessons.

Japanese has two types of adjectives:

i-adjectives all end in い, like おいしい, "delicious." These adjectives are awesome. Think of these as "lovely-ee-ee i-adjectives."

na-adjectives (usually) don't end in い. They also have identity crises and act as nouns sometimes. Think of these as "evil adjectives."

We like i-adjectives because they can form sentences all by themselves. They don't even need copulas like だ in casual sentences. You can see a whole bunch of these 1-word sentences in these two lessons of ours:

[NDL #200] - Super-Duper-Delicious
[NDL #201] - Super-Duper-Delicious, 第２！

Prep yourself before you wreck yourself.

You should know or learn all of these words before reading the rest of this lesson (unless you're just going to skim over everything like a lazy... uh... student)...

i-adjectives:

早い
はやい
early; quick

短い
みじかい
short

小さい
ちいさい
small

安い
やすい
cheap; inexpensive

na-adjectives:

静か
しずか
quiet

Nouns:

ジャム
jam (e.g. "strawberry jam")

空
から
emptiness; blank
Note: When pronounced 空 (そら), this word means "sky."

Verbs:

する
to do; to make

起きる
おきる
to get up; to wake up; to occur

Morphing i-adjectives into adverbs.

To make an i-adjective into an adverb, we just drop off the final い and replace it with く...

Adjective:
早い
はやい
early; quick

Adverb:
早く
はやく
quickly; soon; early

Usually an adverb like this will come directly before a verb:

早く起きる
はやく　おきる
to get up early
Literally: "early + get up."

Grammar Time

JLPT N5: ～くする

Step #1 - Take an i-adjective like like 短い (みじかい // short).

Step #2 - Make it into an adverb: 短く (みじかく).

Step #3 - Add する.

Those three steps will make a phrase that means "make (something) i-adjective."

Like this:

短くする
みじかく　する
to make short

Let's see that in a full sentence:

前髪を２センチぐらい短くしてください。
まえがみ　を　にセンチ　ぐらい　みじかくして　ください。
Please cut [make] my bangs about two centimeters shorter.
Literally: "bangs + を + 2 centimeters + approximately + short + do + please."

I wish we could say that in English. Yo, hair lady, do short 2 centies off the top for me, will ya?

I thought that was really funny... I need to get out more...

Moving on...

Stuff to note about this grammar point:

★ It is used to describe a willful change to something. In English, this often corresponds to the phrase "make + adjective." For example, "make spicy," "make smaller," "make better," etc.

★ Because it is used with willful changes, the verb after ～く will always be a transitive verb. If you don't know what a transitive verb is, don't fret, we'll get to it eventually. (Hint: する, which we'll be using in our examples, is a transitive verb.) You should also read this lesson: [NDL #14] - Knock-Down, Pass-Out Japanese.

★ Though it's very common to have an i-adjective (made into an adverb) before the verb, we can also have nouns and na-adjectives. We'll talk about these later in the lesson.

Examples

ひどい肥満だったから、手術で胃を小さくしました。
ひどい　ひまん　だった　から、　しゅじゅつ　で　い　を　ちいさくしました。
I was morbidly obese, so I had surgery to make my stomach smaller.
Literally: "severe + obesity + was + because, + surgery + で + stomach + を + small + did."

たくさん買うから、もうすこし安くしてくれませんか。
たくさん　かう　から、　もうすこし　やすくして　くれませんか。
I'm going to buy many of them, so could you possibly lower the price (=make them cheaper)?
Literally: "many + buy + because, + a little more + cheep + do + could you please (～てくれませんか).

Now it gets tricky.

Well, not tricky, I guess. Just different.

For i-adjectives: ～くする
For nouns and na-adjectives: ～にする

So all we have to do is put に after a noun or na-adjective to form the same type of sentence.

For example, let's look at a sentence with the na-adjective 静か (しずか // quiet):

周りの人に迷惑だから静かにしなさい。
まわりのひと　に　めいわく　だから　しずか　にしなさい。
Quiet down (=make quiet); you'll bother these other people.
Literally: "surrounding people + に + bothersome + だ + because + quiet + に + do [command form]."

庭でとれたブルーベリーをジャムにしました。
にわ　で　とれた　ブルーベリー　を　ジャム　に　しました。
I used the blueberries I picked from the garden to make jam.
Literally: "garden + で + removed / took off + blueberries + を + jam + に + did."
Note: A more literal translation of the end of the sentence would be "made the blueberries into jam."

金属探知機のゲートをくぐる前に、ポケットを空にしてください。
きんぞくたんちき　の　ゲート　を　くぐる　まえ　に、　ポケット　を　から　に　して　ください。
Please empty (=make empty) your pockets before passing through the metal detector.
Literally: "metal detector + の + gate + を + pass under/through + before + に, + pocket + を + empty + に + do + please."

Those sentences might seem long and intimidating. If so, maybe try matching each word to the literal translation, then go back to the beginning and read through them again (a few times).

Happy studies, fellow student. Or at least, pain-free studies?

No? Low-pain studies? *_*

Bonus Sentences:

空はなんで青いの？
そら　は　なんで　あおい　の？
Why is the sky blue?
Literally: "sky + は + why + blue + の?"

今朝はいつもより早く起きました。
けさ　は　いつも　より　はやく　おきました。
I woke up earlier than usual today.
Literally: "this morning + は + always + more than + early + got up / woke up."