Learn Russian: Не успел он отключить интернет, как ударила молния. - No sooner had he disconnected the internet than lightning struck.

The verb успеть literally means "to have time" or "to manage to do something". In this specific construction (не успел + infinitive + как + past tense), it acts as a fixed phrase translating to "no sooner had... than" or "barely... when". It emphasizes that the first action was barely finished when the second one suddenly happened.

Why is the perfective verb отключить used after не успел instead of the imperfective отключать?

In this "no sooner had" pattern, the first action has to be a completely finished event for the second event to immediately follow it. Because you are talking about the successful, completed result of disconnecting the internet, you must use the perfective infinitive отключить.

Why is как used to connect the clauses instead of a word like когда (when) or чем (than)?

This is simply a fixed requirement of this grammar pattern. While English uses "than" after "no sooner" or "when" after "barely", Russian strictly pairs не успел... with как... to trigger the meaning of an immediate, sudden sequence.

Why is the word order in the second clause ударила молния (verb-subject) instead of молния ударила?

In Russian, placing the subject at the end of the sentence often marks it as the newest, most important, or most dramatic piece of information. Since the sudden lightning strike is the surprise twist of the sentence, putting молния at the very end builds tension and sounds much more natural to a native speaker.