A verb is in the subjunctive mood when it expresses a condition which is doubtful or not factual. It is most often found in a clause beginning with the word if. It is also found in clauses following a verb that expresses a doubt, a wish, regret, request, demand, or proposal.
These are verbs typically followed by clauses that take the subjunctive: ask, demand, determine, insist, move, order, pray, prefer, recommend, regret, request, require, suggest, and wish.
虚拟语气在条件句中的用法
   1、条件从句与现在事实不一致，其句型为：If 主语＋过去时，主语＋should（could, would, 或might）＋动词原形，如：
   If I were you, I would study hard.
   If it rained, I would not be here now.
   2、条件从句与过去事实不一致，句型为：If 主语＋had＋过去分词，主语＋should（could, would, 或might）＋have＋过去分词，如：
   If the doctor had come last night, the boy would have saved.
   If I had not studied hard, I would have failed in the exam last term.
   3、条件从句与将来事实不一致，句型为：
   If主语＋should/ wereto＋do，主语＋should（could…）＋ 原形 do，如：
   If it should rain tomorrow, we would stay at home.
   If I were to go to the moon one day, I could see it with my own eyes.
   If you missed the film to night, you would feel sorry.
名词性虚拟语气：
   在表示命令、建议要求、惊叹时的名词性从句中需用虚拟语气，基本句型：主语＋（should）＋动词原形，如：
   Mother insisted that John go to bed at 9 o’clock.（宾语从句）
   We suggested that the meeting should not be held.
   It was required that the crops should be harvested at once.（主语从句）
   The suggestion that he be invited was rejected.（同位语从句）
   That is their demand that their wages be increased.（表语从句）
虚拟语气在一些特殊词中的使用或含蓄条件句
   1、wish后的宾语从句：
   与现在愿望不一致 主语＋过去时；
   与过去愿望不一致 主语＋had＋过去分词；
   I wish I were you.
   I wish I had visited the white House when I was in the states.
   3、If only引起的感叹句相当于“How I wish＋宾语从句”
   If only he could come! 他要能来就好了。
   If only I had known the answer! 我要早知答案就好了。
   4、would rather, as if（though）引导的句子也需使用虚拟，表示过去的情况用过去完成时，表示现在与将来的情况用过去时，如：
   I’d rather you had returned the book yesterday.
   She loves the children as if they were hers.
   5、without, but, but for, otherwise引起的短语或句子常暗含着含蓄条件。
   Without you, I would never know him.
   But for your cooperation, we would not have done the work so well.