Grammer

This sentence checks if the verb tenses and word order are used correctly.

Explanation:

1. Correct Verb Tense: The sentence starts with "Hardly had he entered," which is in the past perfect tense. So, the next action should be in the simple past tense — "heard," not "was hearing."

So, we say: "Hardly had he entered the room when he heard the explosion."

2. Word Order: The sentence uses an inverted structure because it starts with the word "Hardly." This is normal in English for emphasis, especially after words like "Hardly" or "No sooner."

Correct Version: "Hardly had he entered the room when he heard the explosion."