Präteritum or Imperfekt is used in German primarily in written texts, novels, stories, and fairy tales.
Formation of verb forms in Präteritum:
Weak verbs form the Präteritum by adding the ending –te to the verb stem: kaufte, machte, suchte.
Strong verbs change their root vowel: sprach, las, sang, trank.
I'll offer you a fable in which you can see the use of Präteritum.
Crane and Wolf
A wolf ate (eats) a sheep, and a bone got (gets) stuck crosswise in his throat. In his fear he asked (asks) for help and promised (promises) great reward for this help.
Then a crane came (comes) and thrust (thrusts) his long beak into the wolf's throat and pulled (pulls) the bone out. Then he demanded (demands) the promised reward. But the wolf said (says): 'You want reward too? You should give me reward for coming out of my throat alive! Thank God that I didn't bite your neck off.'