以下是因无法参加活动而写的100字左右英语道歉信模板，分两种常见场景说明：

一、因突发情况无法赴约（如家人突发疾病）

Dear [姓名]，

I am truly sorry to inform you that I cannot attend your [活动名称] on [具体日期] due to [突发情况，如家人突发疾病]。

I understand the importance of this event and apologize for any inconvenience caused. I hope you can forgive me.

Thank you for your understanding.

Best regards,

[你的名字]

二、因已有其他安排无法赴约（如已有承诺）

Dear [姓名]，

I regret to inform you that I cannot keep my promise to attend your [活动名称] on [具体日期] as I have other commitments.

I hope you can understand my situation. Thank you for your patience.

Sincerely,

[你的名字]

注意事项 ：

1. 说明具体原因时需简洁明了，避免模糊表述；

2. 表达歉意时态度诚恳，可适当提及补偿措施（如改期邀请）；

3. 根据活动性质调整措辞，如正式场合可使用更书面化表达。