"That the Earth is round" vs "The earth being round"

Question

A friend of mine recently got this question wrong on a test, but the teacher apparently wasn't able to explain why they got it wrong (at least not clearly). The question is:

__________ is a known fact today, but it was considered a dangerous lie when it was proposed.

1. The Earth is round
2. That the Earth is round
3. The Earth being round
4. Knowing that the Earth is round

The correct answer is (2), but most students found (3) to be an acceptable answer as well. Personally I also found (3) to be acceptable, though I instinctively think (2) is a better answer, which is of no help to my friend.

Can someone explain exactly what is grammatically wrong with the other choices?

Answer 1

That the earth is round is a known fact today, but it was considered a dangerous lie when it was proposed.

As you already know that a finite subordinate clause headed by a subordinator that can function as a complement in a sentence - either a subject or an object.

And hence your sentence

That the earth is round is a known fact today, but it was considered a dangerous lie when it was proposed.

is correct.

As for your first choice The Earth is round, it's a finite clause. But we need some kind of connector word to add it to the other part of the sentence. And, hence, we can't use it as it is.

The earth is round is a known fact today, but it was considered a dangerous lie when it was proposed. [INCORRECT]

If we use your fourth choice Knowing that the Earth is round, the subject position is realised by a Gerund-Participle form of verb - knowing - along with it's own complement - that the earth is round. But it would not mean what we want it to mean. We want to mean the known fact = the earth is round, but with this choice what we would end up expressing is that the known fact = People or anyone for that matter (not specified) knows that the earth is round. So we would not use this choice as well, because it would mean something different.

Knowing that the earth is round is a known fact, but it was considered a dangerous lie when it was proposed. [CORRECT, but don't express the desired meaning]

Your third choice is The Earth being round.

The earth being round is a known fact today, but it was considered a dangerous lie when it was proposed. [CORRECT]

The subject position of the sentence is realised by a Gerund-Participle clause with an explicit subject - The earth being round. Gerund-Participle clause is - being round, and the explicit subject of the Gerund-Participle clause is The earth. The Gerund-Participle clause with its explicit subject mean that the earth is round.

We know that a Gerund-Particple clause can function a number of ways in a sentence, among them is the function of complement in a sentence, a subject.

So your sentence

The earth being round is a known fact today, but it was considered a dangerous lie when it was proposed. [CORRECT]

is grammatically correct. And it gives the desired meaning as well.

Similar sentences -

People being afraid of ABC is the reason I made the movie.

Of course, it is possible that the earth being round is a deep-seated belief.

It is a fact about the world in the same way that the Earth being round is a fact about the world.

Answer 2

It goes without saying that the missing part of the sentence is the subject.

In English, the subject can be realized in the following forms:

Noun (phrase) or pronoun:

The large car stopped outside our house.

A gerund (phrase):

His constant hammering was annoying.

A to-infinitive (phrase):

To read is easier than to write.

A full that-clause:

That he had traveled the world was known to everyone.

A free relative clause:

Whatever he did was always of interest.

A direct quotation:

I love you is often heard these days.

Zero (but implied) subject:

Take out the trash!

An expletive:

It is raining.

A cataphoric it:

It was known by everyone that he had traveled the world.

(The source is here.)

From all the options you have, #1 (a that-clause) is undoubtedly correct, and #2 (as a direct quotation) might also be chosen provided that there wasn't the option #1.

"Knowing that the Earth is round" as a gerund phrase, might also be the subject of a sentence, but when followed by "is a known fact today" seems to be odd.

Compare:

Knowing that the Earth is round won't help you in the job you're applying for.

As for "The Earth being round", it's an absolute participle construction which can't act as the subject of a sentence.