Which past tense do you use when?

In German, there are 3 ways of forming the past tense. The easiest 2 are the Perfect and Imperfect, and there is another slightly more complex form called the Pluperfect. As a quick summary, the Perfect tense refers to actions that occured at a specific point in time, once and finished, for example I saw the Reichstag -> Ich habe den Reichstag gesehen. The Imperfect tense however refers to actions that were ongoing, or happened very frequently, a good example to use for this is "when I was younger ..." -> als Ich junge war. The easiest way to remember which form to use when, is to visualise the past as a river. The flowing river itself is continuous and doesn't necessarily have a specific beginning or end, which symbolises the Imperfect tense. Across the river, picture bridges. These bridges have a beginning and end, and occur at a specific points along the river, these symbolise the actions in the Perfect tense. The Pluperfect is only used when you are already talking about a past event, either in the Perfect or Imperfect, and you want to refer to something that occured before this. For example, if you say "Ich habe den Reichstag gesehen" - using the perfect - but you want to talk about an action that preceeded that, you would say "... aber früher hatte ich das Brandenburger Tor gesehen". This tense is formed in a similar way to the Perfect, but instead of using the present tense conjugation of haben as an auxillery verb, you use the imperfect tense conjugation, hatte, hattest, hatte, hatten, hattet, hatten, followed by the past participle.