It was arranged as he suggested, though Miss Harrison excused herself from leaving the bedroom, in accordance with Holmes's suggestion. What the object of my friend's maneuvers was I could not conceive, unless it were to keep the lady away from Phelps, who, rejoiced by his returning health and by the prospect of action, lunched with us in the dining-room.The Naval TreatySherlock Holmes Illustrated and Complete
Because there is an implied "if" in the word "unless" and "if-clauses" use the subjunctive in conditional sentences. org/dictionary/english/unless
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Because there is an implied "if" in the word "unless" and "if-clauses" use the subjunctive in conditional sentences.
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https://fraze.it/n_search.jsp?q=%22unless+it+were%22&l=0&t=0&ffo=false&findid=-1&ff=
You can substitute "was", but it sounds classier with "were".
CJ