【题目】我想不明白的是为什么他老是上课迟到。
I can't understand is he is always class.

【答案】What；why；late for
【解析】what引导主语从句，从句中作宾语；why引导表语从句与is构成系表结构；be late for“迟到”是常用表达。故填(1). What (2). why (3). late for。