1. No sooner had he come to the office than the phone rang.
Does this mean that the phone rang immediately after he got into the office.
2. No sooner had I answered your question than one of the teachers posted as well.
Does this mean that the teacher posted immediately after I did it.
G. "No sooner did he come to the office when the phone rang."
Could be that the past perfect must be used for the first action and the simple past for the second so as to indicate that the two actions didn't happen at the same time but one immediately after another.
Student or Learner