I hate saying something like "Let's make adjective into nouns," because hearing that usually makes me want to run away as quickly as possible.

But, meh, that's life. Let's face our fears.

Consider these sentences:

1) You run fast.
2) What is the speed of an airplane?

"Speed" is a noun, and "fast" is, in this case, an "adverb."

Why? Because blah, blah, grammar, English, blah, etc.

In Japanese, these sentences will look like this:

1) (Your) running is fast.
2) How much is an airplane's fastness?

None of these make sense to me. Honestly, I'm amazed that I can form sentences in any language. When did that happen?

Here's the real Japanese:

1)
走るの速いね。
はしる　の　はやい　ね。
You run fast.
Literally: "run + の + fast + ね."
Note: Depending on the context, this could also be "He/she runs fast."

走る is "to run," a verb. But when we place a の after it, it becomes a noun, 走るの: "running."

We don't need to say anything for "You" (or whoever we're talking about), because the nuance of this sentence is that we just saw someone run. In other words, it's obvious who the subject is because of context.

Now let's look at...

2)
飛行機の速さってどれくらい？
ひこうき　の　はやさ　って　どれくらい？
What is the speed of an airplane? // How fast do airplanes go?
Literally: "airplane + の + speed + って + how much?"

If we look closely, we can see that the adjective 速い（はやい // fast; quick）has been morphed into the noun 速さ（はやさ // speed）.

I've heard those who call it... grammar!

You take an i-adjective:
速い　（はやい // quick; fast）
長い　（ながい // long）
大きい　（おおきい // big）
厚い　（あつい // thick）
重い　（おもい // heavy）
難しい　（むずかしい // difficult; hard）
寒い　（さむい // cold）
広い　（ひろい // wide; spacious）

Then you chop off the い：
速　（はや）
長　（なが）
大き　（おおき）
厚　（あつ）
重　（おも）
難し　（むずかし）
寒　（さむ）
広　（ひろ）

Then, to make these into nouns, you add さ：
速さ　（はやさ // speed）
長さ　（ながさ // length）
大きさ　（おおきさ // size [lit. big-ness]）
厚さ　（あつさ // thickness）
重 さ　（おもさ // weight [lit. heaviness]）
難しさ　（むずかしさ // [degree of] difficulty）
寒さ　（さむさ // coldness）
広さ　（ひろさ // extent; spaciousness [lit. wideness]）

---

髪長いね。
かみ　ながい　ね。
Your hair's so long.
Literally: "hair + long + ね."

ベッドの長さ測って。
ベッド　の　ながさ　はかって。
Will you measure the length of the bed?
Literally: "bed + の + length + measure."

---

大きい机欲しい。
おおきい　つくえ　ほしい。
I want a big desk.
Literally: "big + desk + wanted."

どれくらいの大きさ？
どれくらい　の　おおきさ？
How big is it?
Literally: "how much + の + size?"

---

厚い本って読むの大変。
あつい　ほん　って　よむ　の　たいへん。
It's hard to read thick books.
Literally: "thick + book + って + read + の + tough/difficult."

このステーキ、すごい厚さ。
この　ステーキ、　すごい　あつさ。
Look at how thick this steak is!
Literally: "this + steak, + amazing + thickness."

---

重いもの運ぶの手伝ってくれる？
おもい　もの　はこぶ　の　てつだって　くれる？
Will you help me carry something heavy?
Literally: "heavy + thing + carry + の + help + give me?"

値段は重さによって変わります。
ねだん　は　おもさ　に　よって　かわります。
The price changes depending on weight.
Literally: "price + は + weight + according to + changes."

---

この本難しい。
この　ほん　むずかしい。
This book is hard.
Literally: "this + book + difficult.

英語と日本語の難しさってどっちが上かな？
えいご　と　にほんご　の　むずかしさ　って　どっち　が　うえ　か　な？
I wonder which is more difficult, Japanese or English.
Literally: "English + and + Japanese + の + difficulty + って + which + が + above + かな？"

---

深いところまで行かないでね。
ふかい　ところ　まで　いかないで　ね。
Don't go in the deep end. // Don't go out to far.
Literally: "deep + place + until + don't go + ね."

海の深さって想像できないよね。
うみ　の　ふかさ　って　そうぞう　できない　よね。
It's hard to imagine the depth of the ocean, isn't it?
Literally: "sea/ocean + の + depth + って + can't imagine + よね."

---

今日寒いね。
きょう　さむい　ね。
It's cold today.
Literally: "today + cold + ね."

冬の寒さ苦手。
ふゆ　の　さむさ　にがて。
I don't like the cold in winter.
Literally: "winter + の + coldness + poor (at)."

---

広い家に住みたい。
ひろい　いえ　に　すみたい。
I want to live in a big house.
Literally: "wide + house + に + want to live."

広さはどれくらいがいいですか？
ひろさ　は　どれくらい　が　いい　です　か？
What size place are you looking for?
Literally: "spaciousness (=wideness) + は + how much + が + good + is + か？"

---

We also have an irregular for いい, "good."

This word is sometimes read/said as よい, and this the form used when "conjugating" いい.

So the past tense is NOT いかった, but instead it is よかった.

Similarly, the noun form is NOT いさ, but instead it is よさ, "merit; value; good quality."

野菜は体にいい。
やさい　は　からだ　に　いい。
Vegetables are good for you.
Literally: "vegetables + は + body + に + good."

スマートホンのよさが全然わかんない。
スマートホン　の　よさ　が　ぜんぜん　わかんない。
I don't get what's so great about smartphones.
Literally: "smartphone + の + merit / good quality + が + not at all + don't understand."

---

We skimped a bit on the notes and breakdowns in this lesson... mostly because I was overwhelmed by the number of example sentences we presented.

Sorry.

_(._.)_
　↑
This is supposed to be a guy bowing. When I type ごめん on my keyboard, one of the options that pops up is _(._.)_

Also: <m(__)m>

I think those < and > are supposed to be arms, and the two m's are supposed to be hands. I'd love to see a Japanese guy actually do that with his fingers when bowing. Like he's going to air-piano the floor to show how sorry he is.