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を見るべきだったのに

should have watched ... but

→ を
phrase Rare #33,921
should have watched ... but Expresses regret or criticism that someone should have watched or seen something, but apparently did not. The phrase typically follows a noun or noun phrase marked by を.
JA: そのドキュメンタリーを見るべきだったのに。
EN: You should have watched that documentary, but you didn't.

君はその展覧会を見るべきだったのに。

You should have seen that exhibition.

そのテレビ番組を見るべきだったのに。

You should have watched that TV show.

そのテレビを見るべきだったのに。

You should have watched that TV show.

あなたは昨晩あの映画を見るべきだったのに。

You should have watched that movie last night.

あなたはその絵を見るべきだったのに。

You should have seen that painting.