There are mistakes you only make once.

There are mistakes you make a couple of times.

And then there are mistakes you make over and over and over again even though your annoying brain knows that you're making a mistake.

That last one happens to me quite a bit. And I always find myself thinking, Really, man? You made that mistake again? That's like the 81,245th time!

An example:

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似合う VS ～に合う

I always always always mix these two words up.

First, definitions:

似合う（にあう // to suit; to match; to look good on [someone]）
合う（あう // to fit; to match; to go well with [something]）

Awfully similar, right?

Let's start with a situation in which I might make a mistake.

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I'm sitting with my friend, and we're eating 餃子 (ぎょうざ // gyoza [potstickers]) together.

My friend says:

A:
ケチャップも意外とイケるよ。
ケチャップ　も　いがい　と　イケる　よ。
Surprisingly they're even good with ketchup.
Literally: “ketchup + も + surprisingly (=unexpected + と) + go well / can do + よ.”

I'm skeptical that gyoza are good with ketchup, but, you know, I'm an open-minded guy. So I give it a shot.

And it tastes good!

Caught up in thoughts of food, my brain doesn't think too much about the whack Japanese I'm using, and a mistake slips out:

B:
ほんとだ！似合う！
ほんとだ　だ！　にあう！
You’re right! They do go well together!
Literally: “true + だ! + suits / matches!”

Boo. Boo. Boo. Wrong.

似合う is typically used for saying that things look good together. As such, it is often used when talking about clothing, hairstyles, and so on.

I could try pretending that I think ketchup looks good on gyoza. Instead, I admit my error and bow gratefully when my friend tells me the correct thing to say:

B:
ほんとだ！合う！
ほんとだ　だ！　あう！
You’re right! They do go well together!
Literally: “true + だ! + fits / matches!”

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When two things are a good fit, then generally speaking 合う is your go-to verb.

I find it particularly useful when talking about foods that go well together. Or don't go well together.

Consider this:

タバスコは和食に合わない。
タバスコ　は　わしょく　に　あわない。
Tabasco doesn’t go well with Japanese food.
Literally: “Tabasco + は + washoku (=[traditional] Japanese food) + に + doesn’t fit / doesn’t match.”

I think the reason that I mix up 合う and 似合う so often is that 合う takes the particle に.

Combined with this particle, the two become homophones: 似合う (にあう) and に合う (に あう).

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Conversely, here are some examples of how you could use 似合う...

大輔はどんなサングラスも似合う。
だいすけ　は　どんな　サングラス　も　にあう。
All sunglasses look good on Daisuke.
Literally: “Daisuke + は + what kind of + sunglasses + も + suit / look good on.”
Note: どんな～も means "anything" or "whatever."

Since we're talking about how something looks, 似合う is the appropriate choice.

Another example:

あたし、ポニーテール似合わないんだよね。
あたし、　ポニーテール　にあわない　んだ　よ　ね。
Ponytails don’t look good on me.
Literally: “I, + ponytail + not suited / don’t look good on (me) + んだよね.”

I can't say for sure whether a ponytail looks good on me or not, as I've never had the courage or patience to get my hair that long.

Maybe someday...

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By the way, note that 合う (あう) means "to match; to fit; to go well with [something]," but when we use different kanji in the word 会う (あう) it means "to meet (someone); to see (someone)."

Our last example:

電車で中学の時の同級生に会った。
でんしゃ　で　ちゅうがく　の　とき　の　どうきゅうせい　に　あった。
I saw a former classmate from middle school on the train.
Literally: “train + で + junior high school + の + time + の + classmate + に + met / saw.”

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After reading this lesson, hopefully you won't find yourself making the same mistakes I always make.

And maybe I'll finally stop embarrassing myself with this one, too.
(↑ Not likely.)