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が最初だ

is the first

→ が
phrase Rare #45,336
is the first が marks what or who is first; the phrase can also be translated as “was the first” depending on context.
JA: この店に来たのは彼が最初だ。
EN: He was the first to come to this store.

ドワネルお前の作文が最初だ

Dwanell, your composition is first.

— 大人は判ってくれない (1959)

スピルバーグの映画を見たのはこれが最初だ。

This is the first Spielberg film I've ever seen.

うちの会社で、コンピューターの世界に飛び込んだのは彼が最初だ。

He was the first person at our company to venture into the world of computers.