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Help the careless time traveler update his official mission log.
If I _________________________ to pack the spare battery, my time machine _______________________________ down in the year 1899.
The correct answers are had remembered and wouldn't have broken.
The third conditional describes a past condition that didn't happen and its imaginary result in the past.
Since the breakdown happened in 1899, it requires the past perfect (had remembered) in the "if" clause and a perfect conditional (wouldn't have broken) in the main clause.