In English grammar, certain phrases are known as correlative conjunctions. They are pairs of words that always go together.

• No sooner... than: This is the standard pair. Because "sooner" is a comparative adjective (ending in -er), it must be followed by "than," just like "faster than" or "taller than."

• Hardly/Scarcely... when: This is a different structure often confused with the one above. If the sentence had started with "Hardly had Tobi left...," then "when" would be the correct choice.