English is to a large extent an uninflected language. This means that there are very few changeable endings to show us the grammatical function of a word. (In fact, it is probably because English is such an uninflected language that it has the reputation of being easy!) Compare this to German, a highly inflected language in which the articles and adjective endings change according to the grammatical function of the noun they qualify. To make this clearer, let's look at an example:
A big dog bit the old man.
In English, neither the articles (a / the), nor the adjective endings change, whatever the function of the noun they go with. In German, however, both article and adjective change according to the function and gender of the noun they qualify. So the same sentence in German would look like this:
Ein grosser Hund biss den alten Mann.
Now you may be wondering what has all this to do with word order, the title of this article. Well, the fact that English is an uninflected language means that the order of words in a sentence must follow a fairly rigid pattern in order to make the meaning clear. First comes the subject (S), then the verb (V), then the object (0).
A big dog (S) bit (V) the old man (O).
If we reverse the position of the two nouns: The old man (S) bit (V) a big dog (O) then we also reverse the meaning of the sentence and it is now the man who is doing the biting. Compare this with German: Den alten Mann (O) biss (V) ein grosser Hund (S). Although we have reversed the position of the two nouns, the meaning has NOT changed. In this sentence the dog is still doing the biting. German allows this change in word order because the endings of the articles and adjectives make it clear what is the subject and what is the object. (To have the man doing the biting, we would have to change the articles and adjectives as follows: Der alte Mann biss einen grossen Hund. Or Einen grossen Hund biss der alte Mann.)
Now this comparison of English and German would seem to prove that English is in fact easier (if less flexible), because the SVO word order is always the same. This would be true, were it not for certain exceptions that make the life of the poor learner of English more difficult. Look at the following examples:
In all the above sentences the verb comes before the subject. This change in word order from the normal is called inversion.
Look at the following pairs of sentences. In each pair, one sentence contains normal SV word order, and the other the inverted VS word order. Decide which sentence is correct. (In one case, both sentences are correct!)
For an explanation of the occasions when inversion is necessary, I suggest you consult a good grammar book such as: Practical English Usage Swan, M Oxford 1997.