Every word in a sentence knows its proper place, and in principle a sentence is an exceedingly orderly affair - a little procession with the verb in the centre and all the other elements arranged around it in rather a strict order.
For example, this is a very proper sentence: "She read it on the bus today." And this one is disorderly chaos: "Reads she on the bus today the paper."
This rigid arrangement works well most of the time, and has the great advantage that the order shows the various roles that the words are playing - the subject is the noun before the verb, the object is the one straight after it, and so on.
But rigid arrangements always create problems, and the same is true in English grammar.
Suppose you've just written this sentence: "She usually reads the paper over breakfast."
Now you want to say she did something different today. You could write our first example sentence, respecting the proper order: "She usually reads the paper over breakfast, but she read it on the bus today."
This sentence would work well in speech, where intonation helps to focus attention on what is important; but in writing it's quite clumsy. Why?
The problem with this sentence has to do with the order of words. The two clauses contrast usual behaviour with today's behaviour, so the word today provides the main link - the reason for putting the two clauses together.
The trouble is that the reader has to read all the other words in the second clause before reaching today, and all that time it remains unclear why this clause is there at all.
It would be much better to start the clause with today: "She usually reads the paper over breakfast, but today she read it on the bus." That flows much more easily, but it's not allowed by the usual rigid word order.
That's why English grammar allows "front-shifting" - a special pattern which overrides the normal order. As you probably guessed, front-shifting allows us to shift things to the front of the clause, and to Hell with the normal rules. It allows almost anything that would otherwise follow the verb to stand right at the front of the clause instead - but only if this helps to link the clause to its context.
For example: "She reads the paper over breakfast, but on the bus she reads her book."
Or even: "She reads the paper over breakfast, but her book she reads on the bus."
Front-shifting is a tool which helps the writer to look after the reader by spelling out the chain of thought; and this tool we think every writer should have.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk