The perfect tenses in English can be a bit tricky, cos they kind of feel like the past tense, and the difference is very subtle, and it takes some getting used to. "The train arrived when the storm began" In the past, the train arrived at the same time as the storm. We're talking about a specific event that orccured at a particular time. "The train had arrived when the storm began" This is very similar, using the past perfect tense. It means something more like "the train arrived at some unspecified time prior to when the storm started".

Hello Gabriela, I believe this exercise is 'Affirmative Sentences' . I think this has less to do with one thing happening before the other, than it does the sequence of tenses. 1. The train arrived when the storm began (The train arrived as the storm began) - Simple past tense The train had arrived when the storm began (The train arrived after the storm) - Past perfect tense 2. The plane had landed when we arrived at the airport (The plane landed after they arrived) - Past perfect tense The plane landed when we arrived at the airport (The plane landed as they arrived at the airport) - Simple past tense 3. The party had begun when her sister left (Party begun after her sister left) - Past perfect tense The party began when her sister left (Party began as her sister left) - Simple past tense 4. The robbers left when the police arrived (Robbers left as the police arrived) - Simple past tense The robbers had left when the police arrived (Robbers left after the police arrived) - Past perfect tense hope I didn't mix anything up, and it may be of some help to You. Best of luck with the homework

1. a) "arrived" and "began" happened at the same time. The train arrived and the storm began around the same time b) The train arrived before the storm (had arrived happed before "began") 2. a) "had landed" happened before "we arrived". When we came to the airport, the plane was already there. b) These two actions happened around the same time. Here you do not want to stress that the plane arrived before. 3. a)Her sister left after the party started. He left in the middle of a party, the guests saw her. b) Her sister left before the party started. 4. a) Police arrived and the robbers left. The robber saw the police arrival, but they managed to escape b) The robbers left before the police came. They did not see the arrival of the police. Police had no chance of catching them.