3) had happened Потому что это Past Perfect. Это время используется там, где одно действие произошло перед другим; в этом случае - Я хочю узнать что случилось с моей машиной( тоесть что то случилось до момента этого разговора(раньше))
Dear John, Sorry for not answering, I was really busy with my homework. Thank you for your letter, I was really happy to hear from you again. Well, as you know, i went to London with my parents last week. Our journey was absolutelly breathtaking! We lived in the hotel near Big Ben. We went to a guided tour. It was really exciting to meet historical places of the city. Moreover, we went sightseeing. We also visited different squers, parks and museums and met with local culture. Sorry, I have to go now. My dad is calling me. Keep in touch! Yours,
I 1) had (lived) in Japan for two years and I 2) had (never experienced) an earthquake before. But all that 3) (changed) one Tuesday morning at work. While I 4) was (sitting) at my desk I suddenly 5) (felt) a small tremor. Before I knew it, my entire desk 6) was (shaking) and my colleagues 7) were (screaming) . At that moment I 8) (remembered) that I 9) had (seen) a safety film about earthquakes the week before. As the film advised, I immediately 10) (ran) to the doorway and 11) (stood) underneath it. After the shaking stopped, I 12) (noticed) that the office windows 13) was (broken) and glass was everywhere. Luckily, no one was seriously injured and we 14) (were) all very relieved it was over!
past perfect употребляется. когда одно действие предшествовало другому. Здесь - что случилось с моей машиной - было по времени раньше