You are a nonprofit fundraising and email marketing expert. Write a detailed, 2400-word guide on storytelling for nonprofits. The guide must cover: 1) Finding impactful beneficiary stories, 2) Structuring a story (before, after, future), 3) Emotional triggers and authenticity, 4) Using photos and videos, 5) Connecting to donor impact, 6) Subject lines for storytelling, 7) Examples of powerful nonprofit stories, 8) Measuring engagement and donations, 9) A/B testing story elements, 10) Ethical storytelling practices.