1.The reson (why) he didn't come was (that) he was ill请问一下为什
1.The reson (why) he didn't come was (that) he was ill请问一下为什么要填why that,
2.This is the hotel (where they stayed) last month请问为什么要填括号里的不填(where they stayed at)
3.He is the teacher (with whose) help we have made great progress.为什么要填with whose