Conversational Past in German
There are actually two acceptable ways of expressing a one-word "simple past" event in German: a) in the German "Imperfekt" (e.g.. ich las) or b) the German "Perfekt" (a.k.a. "conversational past") (e.g., Ich habe gelesen). The German Imperfekt is strictly seen the only formally correct way to describe an even/action that has occurred and ended in the past. (Picture an old school German teacher with a stern look); however, people tend to use the two-word "Perfekt" (especially further South) to express the very same thing. Thus: All English four simple past constructions (i.e., 1) I was reading, 2) I used to read, 3) I read, and 4) I did read) can be expressed in either one way (a) - "Ich las" or (b) "Ich habe gelesen", but from German to English, only one is possible, i.e. "Ich las" cannot be "I have read".
Dear Julika, I am studying German with a teacher and simultaneously using Duolingo to expand my vocabulary and drill it. What I was taught about the use of Preateritum and Perfekt in German is following: 1. Ich ging - Would be used in written communication. 2. Ich bin gegangen - Would be used in oral communication. There is no resemblance to the usage of simple past and other English past constructions in modern German language. Feel free to correct me if I got it wrong.
Link to Wikipedia source: http://en.wikipedia.org/wiki/Preterite#German