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のは初めてだ

it's my first time, it's the first time

→ の
phrase Uncommon #18,893
it's my first time Follows a verb or clause to say that something is being experienced for the first time. It can also be used in the negative のは初めてではないです, meaning “it's not my first time.”
JA: 日本に来るのは初めてだ。
EN: It's my first time coming to Japan.

ドアが閉まってたのは初めてだ

It was the first time I'd ever seen the door closed.

— ファイト・クラブ (1999)

あんなことを人に言われたのは初めてだ。

That's the first time anyone's ever said something like that to me.

こんなに具合が悪いのは初めてだよ。

I've never felt this sick before.

こんなに雪が降ったのは初めてだよ。

I've never seen this much snow before.

彼のような人に平等に扱われるのは初めてだ

It's the first time I've ever been treated as an equal by someone like him.

— どん底作家の人生に幸あれ! (2019)

キスしてもらったのは初めてだ

This is the first time I've ever been kissed.

— ほんとうのピノッキオ (2019)

雪がこんなに積もったのは初めてだ。

I've never seen this much snow accumulate before.

サンチョよこれほど麗しい言葉を聞くのは初めてだ」

Sancho, I’ve never heard such beautiful words before.

— 惑星ソラリス (1972)

phrase Rare #20,611
it's the first time Used after a verb in dictionary form or a noun phrase to say that doing something or experiencing something is a first. The ending may change, such as のは初めてです or のは初めてなんです.
JA: この料理を作るのは初めてだ。
EN: It's my first time making this dish.

ここで食事するのは初めてだよ。

This is my first time eating here.

君たちの中に入るのは初めてだ。

This is my first time joining you guys.

— 教皇選挙 (2024)

殺しを頼まれたのは初めてだ

This is the first time I’ve ever been asked to kill someone.

— グッドフェローズ (1990)

この職にボッピからつくのは初めてだ

This is my first time taking this job through Boppi.

— エリート・スクワッド ブラジル特殊部隊BOPE (2010)