This sentence assesses the accuracy of verb tenses and word order.
Analysis:
1. Verb Tense Accuracy: The sentence initiates with "Hardly had he entered," employing the past perfect tense. Consequently, the subsequent action necessitates the simple past tense—"heard," rather than "was hearing."
The correct construction is: "Hardly had he entered the room when he heard the explosion."
2. Word Order Convention: The sentence employs an inverted structure due to its commencement with "Hardly." This inversion is standard in English for emphasis, particularly following terms such as "Hardly" or "No sooner."
Corrected Sentence: "Hardly had he entered the room when he heard the explosion."