Take a simple sentence such as “The moon is made of cheese.” Here truth certainly matters, and we all jump in with our superior knowledge to shout out that it’s wrong. This is presented as a statement of fact, and if (as authors) we had been astronomers you might (possibly) have believed us.

Now turn this same sentence into a subordinate clause:

“Not many people think the moon is made of cheese.”

Hey presto, no truth - at least, we’re no longer claiming that the moon is made of cheese. The only question of truth is whether many people think so.

Does truth always go out of the window in subordinate clauses? No, it’s more subtle than that, and the point is that a writer needs to be aware of the subtleties.

First, it depends on the verb in the main clause. Take know, for instance:

“Not many people know the moon is made of cheese.”

Truth is back in focus - you can only “know” something that’s true. This is a surreptitious way of announcing that the moon is made of cheese - a claim about the truth. Adjectives can have the same effect:

“I’m glad the moon is made of cheese.”

Or even: “I’m not glad the moon is made of cheese.”

It doesn’t matter whether the main clause is positive or negative; the mere presence of “glad” is enough to guarantee that the subordinate clause is true.

Even clearer grammatical evidence comes from nouns, such as “fact”. These can be combined with a subordinate clause in various ways, but the most useful is the pattern “the fact that I”, which is a noun phrase:

“The fact that the moon is made of cheese is important.”

Here the word “fact” guarantees the truth of the subordinate.

How come, then, that so many people say and write things like this?:

“Scientists have just disproved the fact that the moon is made of cheese.”

This is a bare-faced contradiction: you can’t disprove a fact. If it’s a fact, it’s true, so you can’t show that it’s false.

What these people - who include a good number of KS3 writers - need to learn is that there are other nouns that can be used instead of “fact”. If you want to deny it, you should call it an “idea” or a “theory” or “hypothesis” or even a “supposed fact”: “Scientists have just disproved the idea that the moon is made of cheese.”

Which sounds to us like an important point for sharpening thinking skills.

Richard Hudson is professor of linguistics at University College, London

Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk