Improve the sentence
Hardly he had reached the station when the train departed.

• The required answer is - Hardly had he reached.

• Inversion:
- Hardly, Scarcely, Rarely, Barely, Never, No sooner, By no means of, At no time, Not one, Not once, Not until, etc. Negative Adverb দিয়ে যদি কোন Sentence শুরু হয় তাহলে সেখানে Inversion হয়।
- অর্থাৎ, ঐ Sentence টি Interrogative structure ধারন করে।

• Structure: Negative Adverb + Auxiliary verb+ Subject + Main verb + ------

• Correct Sentence: Hardly had he reached the station when the train departed.

Source: A Passage To The English Language, S. M. Zakir Hussain.