What does it mean? Well, let’s do a little experiment: remove that, and see how the sentence’s meaning is affected: I know she loves me.

Effect: zero. The two sentences have precisely the same meaning.

Conclusion: that has no meaning.

And yet that is one of the 20 most common words in English.

Why? Why do we bother to use a word that has absolutely no meaning at all? And since this is obviously a matter of grammar, what about the idea that grammar is the servant of meaning?

Both questions have satisfying answers, but we have to lay some grammatical foundations first.

There are two words spelt that:

* the determiner which contrasts with this and which always rhymes with hat, as in that book (contrasting with this book);

* the conjunction which generally rhymes with unstressed but (although we can make it rhyme with hat if we stress it). This is the one we’re talking about.

The determiner that has plenty of meaning; it’s the conjunction that is mysteriously empty. If a conjunction is no good for carrying meaning, what is it good for? You’ve guessed it: for conjoining. It’s a signal that the next few words belong to a new clause which is part of a larger clause - in other words, it signals the start of a subordinate clause.

For example, that she loves me is part of the larger clause I know that she loves me. What do I know? That she loves me (or: the answer, or the time, or why she loves me, orI, depending on what follows I know).

Why then do we bother to use the conjunction that? Because this sometimes helps the listener or (more often) the reader. You didn’t need much help when you read I know she loves me. But how about this: I know for sure she loves me.

Wouldn’t it be better to use that: I know for sure thatI? Or this: The really unexpected news she loves me left me walking on air.

Again we think that would help: I news that ... Where syntax is at all complicated, signposts are needed; that is an important signpost.

As writers, we have a duty to signal subordinate clauses clearly, and our students will enjoy the process of exploring the effects of this easily ignored word. Collect examples, real or made up, of sentences which can be made clearer by adding that. For example:

I hear cows moo.

I can see you like Pat.

I know all the pupils in the class who come to me on Monday afternoon will pass.

Our students will learn that small words can have a big impact on meaning.

Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk