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を理解するのは困難

it is difficult to understand

→ を
phrase Rare #39,657
it is difficult to understand The particle を marks what is difficult to understand. The phrase can be followed by forms of だ or である to make a complete statement.
JA: この説明を理解するのは困難だ。
EN: It is difficult to understand this explanation.

私が彼のことを理解するのは困難だ。

I find it difficult to understand him.

彼の言ってる事を理解するのは困難だ。

It's difficult to understand what he's saying.

彼女が言っていたことを理解するのは困難であった。

It was difficult to understand what she was saying.

彼の言っている事を理解するのは困難だった。

It was difficult to understand what he was saying.