As I understand it:
(1) If an engineer were here, he would explain it. Unreal. There is
no engineer here.
(2) If an engineer was here, he would explain it. (Same as No. l.
Popular English use of "was" instead of the "correct" were.
(3) If an engineer had been here, he would have explained it.
An unreal situation in the past.
Student or Learner