撰写一封80词左右的英语道歉信时，关键在于简洁明了地表达歉意、说明原因，并提供弥补措施。以下为详细步骤：

1. 开头：直接表明写信目的，如“Dear [对方称呼], I am writing to apologize for [具体错误]”。
2. 说明原因：简要解释错误发生的原因，例如“Due to [原因]， I failed to [具体行为]”。
3. 表达歉意：真诚地表达歉意，如“I deeply regret this mistake and apologize for any inconvenience caused”。
4. 弥补措施：提出补救方案，如“I will [具体措施] to ensure this does not happen again”。
5. 结尾：请求原谅并表达改进意愿，如“I hope you can forgive me and I will work harder to [改进方向] in the future.”

例如：
“Dear [对方称呼],
I am writing to apologize for missing our meeting yesterday. Due to a sudden family emergency, I was unable to inform you in advance. I deeply regret this mistake and apologize for any inconvenience caused. To make up for it, I will [具体措施] and ensure this does not happen again. Thank you for your understanding.”

通过以上结构，你可以在80词左右清晰表达歉意，并展现诚意。