connective N2 uncommon casualpolitewritten

第一 — first of all / for one thing / to begin with

第一・だいいち

Meaning

first of all / for one thing — leads off the most important point or reason in an argument

そんな計画けいかくは無理むりだ。第一だいいち、お金かねがない。

That plan is impossible. For one thing, we have no money.

Attaches to	Form	Example
Adverb / sentence-initial connective	第一だいいち（に） + clause	第一だいいちに大切たいせつなのは〜

When: Often introduces a clinching first reason against something just said; can also mean 'above all, most important' (第一だいいちに健康けんこうが大切たいせつだ).

外食がいしょくは第一だいいちに高たかいし、栄養えいようも偏かたよる。

Eating out is, first of all, expensive, and the nutrition is unbalanced too.

彼かれに任まかせるのは心配しんぱいだ。第一だいいち、経験けいけんがなさすぎる。

I'm worried about leaving it to him. For one thing, he has far too little experience.

Also used as a noun-modifier 'top / number-one' (第一だいいちの目標もくひょう 'the foremost goal'); the connective use here heads a clause.

Jengo shows 第一 the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.

Sources Compiled with reference to A Dictionary of Advanced Japanese Grammar.