Invert The Subject And Verb With Conditionals

In certain conditional structures, the subject and verb may also be inverted. This can occur when the helping verb in the conditional clause is had, should, or were, and the conditional connector if is omitted.

If he had taken more time, the results would have been better.
Had he taken more time, the results would have been better.

I would help you if I were in a position to help.
I would help you were I in a position to help.
If you should arrive before 6:00, just give me a call.
Should you arrive before 6:00, just give me a call.

In each of these examples you can see that when if is included,
the subject and verb are in the regular order (if he had taken, if I were, if you should arrive). It is also possible to omit if; in this case, the subject and verb are inverted (had he taken, were I, should you arrive).

The following example shows how this sentence pattern could be tested in structure questions on the TOEFL test:

Example from the Paper and Computer TOEFL® Tests
The report would have been accepted _____ in checking its accuracy.
(A) If more care
(B) More care had been taken
(C) Had more care been taken
(D) If had more care been taken

In this example, a connector if and a subject and verb are needed,
but if could be omitted and the subject and verb inverted. Answer (A) is incorrect because it contains the connector if and the subject care but no verb. Answer (B) is incorrect because it contains the subject care and the verb had been taken but does not have a connector. In answers (C) and (D), if has been omitted. Because it is correct to invert the subject more care and the helping verb had, answer (C) is correct.