Two tenses, one meaning
For almost every verb, the Perfekt and the Präteritum are both correct and mean the same. English is different: I ate closes a finished stretch of time, I have eaten keeps it open into now. German does not make that distinction. The choice depends on where you use the sentence.
The same event, twice
Ich habe gestern meinen Pass verloren.
I lost my passport yesterday.
Perfekt: spoken, completely normal with gestern.
Ich verlor gestern meinen Pass.
I lost my passport yesterday.
Präteritum: the written version.
Which tense where
Perfekt: spoken
Conversation of every kind, formal or not.
Messages and emails that sound like talking.
Dialogue inside a novel, because the characters are speaking.
Was hast du am Wochenende gemacht?
What did you do at the weekend?
Wir sind einfach zu Hause geblieben.
We just stayed at home.
Präteritum: written
Novels, short stories, fairy tales: the narrator's voice.
Newspaper reports and news read from a script.
Reports, minutes, and historical texts such as Wikipedia.
Der Verletzte kam ins Krankenhaus.
The injured man was taken to hospital.
Sie studierte in Freiburg und arbeitete danach in Zürich.
She studied in Freiburg and afterwards worked in Zurich.
Ask yourself whether a voice is saying this or a narrator is telling it. A journalist writes Die Polizei nahm den Mann fest. The same journalist, telling a friend at dinner, says Die Polizei hat den Mann festgenommen. The event is the same. Inside one written text, stay in one tense.
Verbs that stay in the Präteritum when speaking
You already know that sein, haben, the modal verbs, es gibt and werden usually take the Präteritum even in speech. A few more verbs join them, mostly in fixed turns: wissen, denken, finden (as an opinion), heißen and es geht um. The Perfekt of these is correct but often sounds stiff.
Ich war noch nie in Wien.
I've never been to Vienna.
English needs the perfect here; German says war.
Das wusste ich nicht.
I didn't know that.
Den Film fand ich super.
I thought the film was great.
Ich wollte dich anrufen, aber ich habe es total vergessen.
I meant to call you, but I completely forgot.
Both tenses in one sentence: normal in speech.
Unlearning the English perfect
English forbids a finished time expression with the present perfect: I have seen him yesterday. German's Perfekt is not the English present perfect. It is simply the past tense you speak in, so gestern, letzten Sommer, 1998 and vor drei Wochen all go with it: Ich habe ihn gestern gesehen.
Wir haben 1998 geheiratet.
We got married in 1998.
Vor drei Wochen bin ich umgezogen.
I moved house three weeks ago.
Letzten Sommer haben wir in Portugal Urlaub gemacht.
Last summer we went on holiday in Portugal.
