As a rule of thumb, Germans use the perfect tense to express the past tense in spoken German, except with certain verbs and except in certain situations. The verbs with which the perfect tense is usually not used (apart from for situations for which the perfect tense is the preferred tense) are the auxiliary verbs, modal verbs and, in Central and Northern Germany, also certain other common verbs. These are used in the simple past instead.
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I've put a list of which verbs aren't generally used in the perfect tense on my geocities site, where I put grammar tables and transcripts of the episodes: http://sites.google.com/site/germangrammarpod/past. The website also includes a table showing the information I've given in my podcasts so far about when to use which tense.
It's always tricky to describe when a tense should be used in a foreign language, and there's a lot of seemingly contradictory information out there. To compile this episode, I mainly used German-language Wikipedia:
http://de.wikipedia.org/wiki/Perfekt and
http://de.wikipedia.org/wiki/Pr%C3%A4teritum
which, slightly disturbingly, both seem to have been rewritten since I used them for information (although a native speaker did recommend the sites at the time I used them, so at least one native speaker did think they were supplying correct information as they were).
I also used the book Hammer's German Grammar and Usage (in my case the second edition). Here's a link to the fourth edition on Amazon: Hammer Grammar, although I recommend any edition of it that you can get your hands on.
I also liked the information in about.com on this topic: http://german.about.com/library/verbs/blverb_past.htm
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