Hi Newguest, For a start, that sentence should really have commas around the inserted relative clause to show that the action that follows still applies to the main subject (experiment) - so: In fact, the reactions of both groups were so intense that the experiment[,] which was to have lasted for two weeks[,] was broken off after six days. Using "was to last" instead of "was to have lasted" sounds slightly strange to me in the new sentence. I think it is because the use of straight "was" sounds more definite, making the part that follows feel contradictory.
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WOULD IT BE OK IF I WROTE .......Yes, but, as discussed above, it would not convey as nicely the counterfactual aspect felt in the original was to have lasted.