I wonder if the indicative present is good in the clause:
it is the requirement that, when John arrives, Mark be here
The answers are detailed but I wish to discuss the case "when becoming if" in order that I understand better: When we wish say "if John arrives at the same time" I believe nothing changes.
it is the requirement that, if John arrives, Mark be here
When we wish to say "if John arrived yesterday", then I do not see why it is not the past subjunctive. Past because it is on yesterday that we are focusing, subjunctive because it could happen or not (because of the if). So,
it is the requirement that, if John (had?) arrived, Mark be here
Is it with had or without ? what would be the difference ?
I changed the title.