In North & Hillard; Ex 191, Q10: the student is required to translate: "He refused to fight until reinforcements came."
An awkward one: the student has to remember to use "negavit" (he denied that); it's indirect speech; here, requiring a reflexive pronoun (coincident subject); "until" = dum (+ subjunctive).
The answer-book confirmed: "negavit se pugnaturum esse dum novae copiae advenirent."
The sequence-of-tenses rule dictates that the future tense ("until reinforcements [will] arrive", is linked to the present subjunctive. Therefore, why use imperf. subj. "advenirent", as opposed to pres. subj. "adveniant"?