Like other kosoado pronouns, we can divide these four words like this:
- kou こう
This way. The way I'm talking about.
In this manner.
- sou そう
That way. The way you said.
In that manner.
- aa ああ
That way. The way neither of us are talking about.
In that manner.
- dou どう
What way? How?
In what manner?
(aa ああ is also a way to say "yes" in Japanese)
In dialogue, kou こう and sou そう refer to manners which the speaker and the listener mentioned respectively. So they're used with far more frequency than the pronoun aa ああ. However, aa ああ is, indeed, believe it or not, an actual Japanese pronoun too. Probably.
The trickiest thing about the pronouns kou, sou, aa is that they are also adverbs grammatically speaking, so they can be connected directly to verbs without any use of Japanese particles. Take a look:
- kou sureba yokattanda こうすればよかったんだ
It was good to have done it like this.
It'd have been better to do it like this.
- naze kou natteshimatta!? なぜこうなってしまった！？
Why did it end up like this!?
- kare ga kou itta... 彼がこう言った…
He said (like this)... (insert phrase here)
- koko wo kou kaetara dou? ここをこう変えたらどう？
What if you change this part like this?
- sou omou そう思う
I think so.
I agree with that.
- sou wa ikemasen そうは行けません
That won't do.
You can't do things that way. (I won't let you)
- min'na aa desu ka? みんなああですか？
Is everyone that way?
- dou yatte katsu? どうやって勝つ？
How to win?
- kono kuni wa dou naru? この国はどうなる？
How is this country going to turn out?
- dou demo ii どうでもいい
It doesn't matter!
Whatever way is good.
- dou mo kou mo nai! どうもこうもない！
None of that is going to work!
(this is an expression used when someone proposes a stupid solution to a problem he probably created in first place)
The words kou, sou, aa and dou are often spoken alone as an abbreviated way to ask certain questions, which can be confusing for people who're learning Japanese. Take a look:
- kou? こう？
kou desu ka? こうですか？
In this way?
Am I doing this right?
- kou! こう！
- sou? そう？
sou desu ka? そうですか？
Is that so?
Is it the way you said?
- sou! そう！
- aa? ああ？
aa desu ka? ああですか？
Is it that way?
(indicating a manner that's not related to speaker or the listener)
- dou? どう？
dou desu ka? どうですか？
How is it?
Is it good? Is it bad? Say something!
(can be used for taste of food, clothing, medical examinations, etc.)