Immer wiederholen!Perfekt or Präteritum?
Perfekt and Präteritum describe the same past — there's almost no difference in meaning. The difference is where you're telling it: it's a question of register, not meaning.
The basic rule
In spoken, conversational German, Perfekt is used almost all the time, no matter when
something actually happened: Ich habe das Buch gelesen. — "I read the book"
(said out loud, whether it was yesterday or last year — the form doesn't change). In
written narration — books, news articles, fairy tales, formal reports — the norm is the
opposite: Er las das Buch. Both sentences mean the same thing; the choice
depends only on whether you're speaking or writing a connected narrative.
The exception: haben, sein, and the modal verbs
For haben, sein, and the modal verbs (können,
müssen, wollen, dürfen, sollen,
mögen), Präteritum is the usual choice even in speech, not Perfekt:
Ich war müde. sounds natural, while the technically correct
Ich bin müde gewesen. is clunky and bookish. Worth memorizing as its own
exception to the "Perfekt when speaking, Präteritum when writing" rule.
Why learn both forms
Even if your own speech mostly sticks to Perfekt, Präteritum is still essential: it's the standard past tense in books, articles, and news — in other words, most of what you'll read. And haben, sein, and the modal verbs make Präteritum common in speech too. The bot drills both forms for the same set of irregular verbs — see "German irregular verbs" for more.
Practice
To make sure it sticks, try a few exercises on Perfekt and Präteritum forms.