Hardly \_\_\_\_ the movie begun when the power suddenly went out in the theater.

Hardly 置於句首時，句子需使用倒裝結構，將助動詞移至主詞前。搭配後方的過去分詞 begun 以及後半句的過去式 went out，應使用過去完成式 had，構成 Hardly had the movie begun 的正確句型。