2. So my biggest question is that (in until sentences) why "the present/past/future perfect” tense in the main clause, their action can be all both ended before the time which ”until clause” mentions. This assumption of yours is incorrect. Their action IS NOT finished before the time the 'until' clause mentions. Example: I had waited until 3pm, but he still had not arrived. This is saying I waited and waited and waited (without ever stopping waiting) UNTIL 3pm. Then I finally stopped waiting. There was NEVER a pause in my waiting before 3pm. In your example: "I had waited until the rain stopped" Here I am talking about the past. I waited CONTINUALLY in this past time until the rain stopped. Then I stopped waiting.

Thank your for your kind reply. i totally understand Greg's explanation. And that is what I thought originally. But after I review the definition of "past perfect tense" and the usage requirement of the "until sentences", I found it is contradictory. Could you please help me out of the conflicting problem? 1. the definition of the past perfect tense: The past perfect is a verb tense used to talk about actions that "Were Completed" before some point in the past. source from https://www.grammarly.com/blog/past-perfect/ 2. "Until" in dictionary: if something happens until a particular time, it "Continues" and then stops at that time from https://www.ldoceonline.com/dictionary/until 3. (1)'The past perfect' indicate an action completed (2) 'until' also indicate the same (past perfect) action completed. a. Isn't it redundant to say it twice that an action is completed? b. So I was confused, why 'The past perfect' whose action is completed can be used in "the main clause of until sentence" which require the action should be "Not" completed and Continuous 4.in Greg's explanation, "I had eaten my dinner until 10p.m." means "I ate my dinner until 10p.m., I had finally eaten my dinner" it's my first time to know the past perfect action can be extended and kept to another period of time. 5. how about "I had eaten and had waited until 10p.m."? Which it will mean as follows? a. i finished eating, and then kept waiting until 10p.m. b. I kept eating and kept waiting (at the same time) until 10p.m. If there is anything wrong, please help to correct me. Thank you.

Greg gave an excellent answer.