[Japanese Grammar] Why is it せず and not しず?

Hi! I’m Mai-sensei, a native Japanese tutor.

If you’ve been studying Japanese for a while, you’ve probably seen sentences like:

• 何も言わずに帰った。

• 連絡せずに来た。

• 文法を勉強せずに話せるようになりたい。

And if you’re a sharp learner (or a chronic overthinker… welcome to the club), you might have asked:

“Wait, why is it せず and not しず?”
Because する → しない, right?
So shouldn’t it be する → しず?

This is actually a fantastic question.
And it reveals something super important:

～せず is not based on modern Japanese grammar.
It comes from older / classical Japanese, and that’s why it looks “irregular.”

But don’t worry. It’s not just random chaos.
There is a reason.

Let’s unpack it.

What is ～せず, grammatically speaking?

The structure of ～せず is:

Verb (Mizen-kei) + ず

Here’s the key term:

Mizen-kei (irrealis form / imperfective form)

“Mizen-kei” is a verb conjugation form used in classical Japanese, often appearing before negative forms.

For now, you only need one idea:

～ず attaches to the Mizen-kei form.

Examples:

• 言う → 言わず

• 書く → 書かず

• 行く → 行かず

• 食べる → 食べず

So far, so good.

Now comes the troublemaker…

Then why isn’t it “しず”?

Because する does not use “し” as its Mizen-kei.

In older Japanese, する had a full conjugation set like this:

• Mizen-kei (irrealis form): せ

• Ren’yō-kei (continuative form): し

• Shūshi-kei (plain form): す

• Rentai-kei (attributive form): する

• Izen-kei (realis form): すれ

• Meirei-kei (imperative): せよ / しろ

So the negative ～ず form becomes:

せ (Mizen-kei) + ず → せず

That’s it.

Not magic. Not a typo. Not a secret conspiracy.

Just… old grammar living rent-free in modern Japanese.

But why “しない” then?

Great follow-up question.

Because ～ない is modern Japanese, and it typically attaches to a different form.

In modern Japanese:

Ren’yō-kei (continuative form) + ない

So:

• する → し + ない → しない

That’s why learners feel confused:

• しない looks like the “normal” negative

• せず looks like the “weird” negative

But they belong to different systems.

Quick summary:

• しない = modern negative (ない)

• せず = classical negative (ず)

Different origins, different rules.

Why does Japanese keep these older forms?

Because language is a museum and a living organism at the same time.

Even though modern Japanese doesn’t actively “use” classical conjugation systems in daily speech, many forms survive in:

• written language

• formal speech

• idiomatic expressions

• set phrases

So ～ず and ～せず remain common, especially in writing.

Final takeaway

If you want a short answer:

• せず is “classical-style negation,” not modern-style negation.
• ～ず attaches to the Mizen-kei form.
• The Mizen-kei of する is せ.
• That’s why it becomes せず.

 

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