第一 | Grammar

Structure

As a prefix ("the first," "number one"):

第一 + [noun]

As a noun expressing priority ("comes first"):

[noun] + が第一だ
[noun] + を第一に（考える／する）
[noun] + 第一（で）

As an adverb ("to begin with," "first of all"):

第一、[sentence]
第一に、[sentence]

Description

第一 (だいいち) has several related uses, all revolving around the idea of "first" or "foremost." It can function as a noun, an adverb, or a prefix depending on context.

"First" / "Number one" – as a prefix or noun

第一 can be attached to nouns to mean "the first" or "number one."

。

First impressions are very important.

。

This is the first step toward success.

"Most important" / "Foremost" – expressing priority

第一 is used to express that something is the top priority or most important thing. The pattern ～が第一だ or ～を第一に is common.

。

Health comes first.

。

Please work with safety first.

。

We put our customers first.

"To begin with" / "In the first place" – as an adverb

When used adverbially (often at the beginning of a sentence or clause), 第一 means "to begin with" or "in the first place." This usage frequently carries a tone of criticism or complaint, pointing out a fundamental flaw or the most important reason why something is wrong.

、。

To begin with, we don't have money.

、。

In the first place, he isn't ready yet.

This adverbial use is similar to そもそも ("to begin with") but 第一 tends to emphasize the most important point among multiple reasons, while そもそも questions the fundamental premise. 第一 is also often followed by additional reasons using 第二 (second), 第三 (third), etc.

Examples

、。

First impressions are important, so wear proper clothes.

。

Let's act with safety first.

。

I believe family comes first.

、。

To begin with, we don't have time, so it's impossible.

。、。

That plan won't work. In the first place, the budget isn't enough.

。

She puts health first in everything.

、。

First of all, let's confirm the goals of this project.

。、。

I'm against it. To begin with, it's too dangerous.

。

This is the first step in learning Japanese.

。

We place the highest priority on customer satisfaction.

、でしょうきごう？

In the first place, you haven't even consulted him, have you?

。。

Studying comes first right now. Save playing for later.

References

第一Bunpro