求大神解释if引导的虚拟语气

if引导 非真实条件句 分3种与现在相反if+主语+动词过去时（be用were）+。,主语+could/would/might/should+动词原形+。与过去相反if+主语+had been+ 动词过去分词。,主语+could/would/might/should+have+动词过去分词。与将来相反if+主语+should/were to+动词原形。,主语+could/would/might/should+动词原形+。