我正在写信告诉你我昨晚缺席的原因
"我正在写信",英语要用现在进行时,即:I am writting (a letter). "告诉你"表示目的,英语要用动词不定式,即:to tell you. "昨天晚上缺席的原因"要作成定语从句修饰"原因",即:the reason why I was absent last night. 所以全句用英语这样写:I am writing to tell you the reason why I was absent last night.