Inversion

AUXILIARY VERB + SUBJECT + MAIN VERB

We put an auxiliary verb before the subject of a clause in several different structures:

1. DIRECT QUESTIONS: Do you like Brussels sprouts?

2. EXCLAMATIONS: Have you got a surprise coming!

3. WITH MAY: May all your wishes come true!

4. AFTER SO, NEITHER, NOR:

In 'short answers' and similar structures, these words are followed by auxiliary verb + subject.

'I'm hungry.' 'So am I'

'I can't swim.' 'Neither / Nor can I.'

5. AFTER NEGATIVE AND RESTRICTIVE EXPRESSIONS:

If a negative adverb or adverbial expression is put at the beginning of a clause for emphasis, it is usually followed by auxiliary verb + subject. These structures are mostly rather formal.

Under no circumstances can we cash cheques.

Not until much later did she learn who her real father was.

The same structure is possible after a complete clause beginning not until... 

Not until he received her letter did he fully understand the depth of her feelings.

Inversion is also used after restrictive words like hardly, seldom, rarely, little and never, and after expressions containing only. These structures, too, are formal or literary.

Hardly had I arrived when trouble started.

Seldom have I seen such a remarkable creature.

Little did he realize the danger he faced.

Never...was so much owed by so many to so few. (Churchill)

Only then did I understand what she meant.

Only after her death was I able to appreciate her.

Not only did we lose our money, but we were nearly killed.

Inversion can occur after another complete clause beginning with only after, only if and only when.

Only after she died was I able to appreciate her.

Inversion is used when not + object is put at the beginning of a sentence for emphasis.

Not a single word did she say.

6. AFTER AS, THAN, AND SO:

She was very religious, as were most of her friends.

City dwellers have a higher death rate than do country people.

So ridiculous did she look that everybody burst out laughing.

7. CONDITIONAL CLAUSES

In formal and literary conditional clauses, an auxiliary verb can be put before the subject instead of using if.

Were she my daughter... (=If she were my daughter... )

8. AFTER ADVERBIAL EXPRESSIONS OF PLACE:

When an adverbial expression of place or direction comes at the beginning of a clause, intransitive verbs are often put before their subjects. This happens usually when a new indefinite subject is being introduced. The structure is most common in literary and descriptive writing.

Directly in front of them stood a great castle.

This structure is often used in speech with here, there and other short adverbs and adverb particles.

Here comes the bus!

9. REPORTING:

In story-telling, the subject often comes after reporting verbs when these follow direct speech.

'What do you mean?' asked Henry.

If the subject is a pronoun, it usually comes before the noun.

'What do you mean?' he asked.

SWAN, Michael. 1995. Practical English Usage. 2º ed. Oxford: Oxford University Press.