How this works
THEREFORE
Consequence. Beat A causes Beat B. The story escalates. "The hero trained for years, therefore she entered the tournament."
BUT
Reversal. Beat A is interrupted or complicated by Beat B. Tension spikes. "She was winning, but the champion bribed the judge."
AND THEN
List of events. No causality, no stake, no reason to keep reading. The audience's investment flatlines. "She trained, and then she entered, and then she won." Fix: add conflict (but) or consequence (therefore).
Source: Trey Parker & Matt Stone (South Park), NYU storytelling lecture — and every working screenwriter since Aristotle.