Frank was about to leave when he noticed a letter lying on the floor.
问题描述：
Frank was about to leave when he noticed a letter lying on the floor.
这话是什么时态,什么结构,
答
这是个复合句,
主句Frank was about to leave 为过去将来时（be about to do表示“将要做某事”）,
从句when he noticed a letter lying on the floor为when 引导的时间状语从句.
整句话的意思是：
弗兰克正要离开,这时他发现地上有一封信.