For instance, that last sentence could have been: "This column is about that." The meaning would have been correct, and - better still - it would even have been true, but it wouldn't have been so helpful to you. And the sentence before it could have been: "We all had to learn when, and how, to make the point crystal clear." Same comments: true, but unhelpful.
The interesting thing about these examples is that helpful grammar is complex. Here are the sentences again:
1. This column is about that.
2. That is what this column is about.
Sentence 2 takes two more words (is what) to give the same information, and yet you'll surely agree that it's more helpful.
3. We all had to learn when to make a point clear.
4. One of the things that we all had to learn was when to make a point clear.
In this case, the more helpful sentence 4 took six more words: "one of the things that ... was". And yet we're sure that these extra words were well used.
What these sentences do is to convert a statement into a kind of question-answer form. (There we go again: a simpler wording would have been: "These sentences convert a statement...") For example, sentence 2 asks what this column is about, and then answers it with that; and sentence 4 asks what we all had to learn before giving the answer. In other words, you start with the simple wording (sentences 1 or 3), and split it into two parts: question + answer. Grammarians call this process "clefting", and the result is called a "cleft sentence".
In slightly more technical terms, clefting takes one part of the simple sentence and turns it into a relative clause. Sometimes this is an ordinary relative clause, such as "(things) that we all had to learn". But it can also be a special kind of relative clause that can be used on its own without a modified noun, as in: "what this column is about". Either way, the relative clause sets up a question which is answered by the remaining part of the simple sentence.
Converting simple sentences to clefts and back again is a really useful classroom activity, as it focuses attention on the importance of context.
Start with: "Billy writes music." They offer: "It's Billy that writes music", "What Billy does well is write music", "What Billy writes is music." The class will learn flexibility - or rather, what the class will learn is flexibility.
Richard Hudson is emeritus professor of linguistics, University College London
Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk