Peter went to buy some glass so he could fix the broken windows.
In situatia data, varianta corecta este some deoarece glass este un substantiv uncountable atunci cand are sensul de materie prima (din care se fac ferestre, etc).
Glass este countable doar atunci care are sensul de pahar (one glass, two glasses). Doar in aceasta situatie putem spune a glass (of .....) si are sensul de un pahar.
