ПОМОГИТЕ С ОБЪЯСНЕНИЕМ. ГДЕ СТАВИТСЯ PAST PERFECT,PAST PERFECT CONTINUOS, PAST

ПОМОГИТЕ С Изъяснением. ГДЕ СТАВИТСЯ PAST PERFECT,PAST PERFECT CONTINUOS, PAST SIMPLE,PAST CONTINUOUS
Dear Sam,
How are you You won't believe what 1)...(happen) to me last Saturday. I 2)...(walk) along the beach with my dog, Sam, when he 3)...(spot) something in the sand. He 4)...(start) digging it up and waving his tail happily. It 5)...(turn) out to be a wooden box with coins in it. I quickly 6)...(grab) in from Sam who 7)...(try) to chew it and 8)...(take) it straight to my history teacher, Mr Brown. As soon as I 9)...(see) the look in his eyes, I 10)...(know) we 11)...(find) something special. Together, we 12)...(go) to the local museum. They 13)...(examine) the coins and they 14)...(announce) that they were 250 years old. Can you believe it A box full of valuable coins 15)...(wait) for someone to find to for so many years The people at the museum 16)...(come) to investigate. They 17)...(dig) all around the area, but they 18)...(not find) anything else. It's still a bit of a mystery how the box 19)...(come) to be there.