Rule reminder
Complete review: anche se + indicative (colloquial/neutral), sebbene/benché + subjunctive (formal), nonostante/malgrado + subjunctive or noun (formal), pur + gerund (same subject, formal), per quanto + subjunctive (degree), quand'anche + imperfect/past perfect subjunctive (literary, hypothetical). Always respect tense agreement and register.
Scegli la versione più stilisticamente ELEVATA: "Even though they had promised to help, they didn't show up."