нулевое-законы природы или общепринятые правила: if you heat water,it boils первый-реальные возможности:if you work hard,you will enter a university второй-реальные возможности в будущем, но невозможны в present(или невозможны в целом:if I were you,I would work hard. третий-невозможны вообще:if he had worked hard,he would have entered the university of his dream last year