Hello HiLine, this is a good observation.
Thesubject of the sentence that you've stated in your post, isWhat is difficult to determine . This is itself aclause .
When the subject of a clause itself is aclause , the subject is consideredsingular .
While not exactly thesame example, a similar official example:
That some fraternal twins resemble each other greatly while others look quite dissimilar highlights an interesting and often overlooked feature of fraternal-twin pairs, namely that they vary considerably
...
Thesubject of the sentence that you've stated in your post, isWhat is difficult to determine . This is itself aclause .
When the subject of a clause itself is aclause , the subject is consideredsingular .
While not exactly thesame example, a similar official example:
That some fraternal twins resemble each other greatly while others look quite dissimilar highlights an interesting and often overlooked feature of fraternal-twin pairs, namely that they vary considerably
...
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