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が、実は

but actually

→ が
phrase Rare #32,461
but actually Introduces a fact that contrasts with or contradicts what came before.
JA: 彼は静かに見えるが、実はよく話す。
EN: He seems quiet, but he actually talks a lot.

彼は教師をしているが、実はスパイだ。

He is a teacher, but in reality, he is a spy.

彼は落ち着いて見えたが、実はとても緊張していた。

He looked calm, but he was actually very nervous.

その男の子は病気には見えないが、実は病気だ。

That boy doesn't look sick, but he actually is.