The word subierais is the imperfect subjunctive form of subir in the second person plural.
See the full subir conjugation

subir

No puedo creer que os subierais al tren. Ni yo.
I can't believe you hopped a freight. Me neither.
Podrían haber esperado a que subierais.
They could have waited till you all got up there.
Su padre intenta hacer algo bonito, Así que me gustaría que dejarais de quejaros, y se subierais al coche.
Yourather is trying to do something nice, So I would appreciate it if you would stop your whining And just get in the car.
Así que me gustaría que dejarais de quejaros, y se subierais al coche.
So I would appreciate if you would stop your whining and just get in the car.
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