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的是，他

is that he

→ 的
phrase Rare #31,653
is that he Follows a phrase describing a reaction or judgment and introduces what caused it or what happened.
CH: 最让我担心的是，他还没有回家。
EN: What worries me most is that he still hasn’t come home.

让我们非常惊讶的是，他突然辞职了。

To our great surprise, he suddenly resigned.

让我出乎意料的是，他居然很会唱歌。

To my surprise, he can actually sing very well.

让事情更糟糕的是，他没有注意到他打扰到了邻居。

To make matters worse, he didn't realize that he was disturbing the neighbors.