Mike ( had broken | broke ) his mother's vase last night but he has not told her yet.
Some argue it must use "broke" instead of "had broken" because "last night" could not be used with past perfect tense. I notice some teachers preach a grammar rule: Adverb like "last night, last year, a few days ago" should not be used with past perfect tense; It should use past tense. But I never find any formal grammar books make that statement. Instead, I think it is quite a "weak grammar rule" which is often "violated" in some writings. I have quite often seen sentences like "He had taken the test last year." and similar ones in popular novels, news papers and magazines.
I think "had broken" is a better answer here as it make the sentence more cohesive in meaning and logic, even though "broke" is ok too, at least grammatically.
Maybe only in cases for a specific time point like "at 8 o'clock", we might have to use past tense. e.g, He broke the vase at 8 pm last night.