Choose the grammatically correct sentence:

The correct answer is Option (4) → Had I known about the power cut, I would have charged my phone last night.

A condition without “if”:

• A condition about the past can start with “Had” in place of “If I had”. Then “if” is dropped and “had” comes before the subject.
• Here Had I known means “If I had known”. The verb after “had” is the past participle (V3) “known”, and the second part takes “would have”.

Why the Other Options are Incorrect?

Option 1: Had known I about the power cut, I would have charged my phone last night. — Incorrect. The option says “Had known I”. Only the helping verb “had” moves before the subject. The V3 “known” stays after it: “Had I known”.

Option 2: Had I knew about the power cut, I would have charged my phone last night. — Incorrect. The option says “Had I knew”. After “had”, the verb takes the V3 form “known”, not the past form “knew”.

Option 3: I had known about the power cut, I would have charged my phone last night. — Incorrect. The option says “I had known ..., I would have”. With no “if” and no change of order, the first part is a plain statement, not a condition. “Had” must come before “I”.