In present-day English, inverting the subject with should, were and had to express the sense of an if clause is still technically 'grammatical':
Should you find my book (= If you should find my book), please send it to me.
Were he running (= If he were running) he might win.
Had she gone (= If she had gone) to the interview she would have been hired.
Had and were may be inverted even when they are not used as auxiliaries:
Were he here (= If he were here), he could show us how to do it.
Had I a brother (= If I had a brother), life would be more enjoyable.
In practice, however, this construction is only found in very formal texts, or in the speech of people trying to sound formal; and even there it has a quaint, old-fashioned ring.
This is "school English": you need to be able to recognize its use in older texts, but there is no reason for you to use it in your own writing or speech.