"Für diesen Preis kann ich zwanzig Äpfel kaufen!"
Translation:For this price I can buy twenty apples!
When the first position in a sentence is taken by something other than the subject (assuming it's a statement rather than a question), the subject and the verb switch places so that the subject comes after the verb. "Kaufen" is unconjugated and last because of the modal verb "können". When a verb phrase has several verbs, only the first verb gets conjugated and the other verbs are moved to the end without conjugation.