I wish it would snow这句话是虚拟语气用于宾语从句,意思是表示不能实现的愿望,LZ应该有个具体环境,所以可以为以下答案:
表示现在不能实现的愿望,从句的谓语动词过去式;
I wish it snowed.
表示将来不能实现的愿望,用“would(could)+动词原形”;
I wish it would snow
虚拟语气的用法总结:
一、虚拟语气用于条件状语从句.
1.表示与现在事实相反的假设时,条件状语从句中的谓语动词用“过去式（be的过去式用were）”.而主句中的谓语动词用“would(should, could, might)+动词原形” .
例如：If he had time, he would attend the meeting.
2.表示与过去的事实相反的假设时,条件状语从句的谓语动词用“had+过去分词”,主句中的谓语动词用“would(should, could, might)+have+过去分词”
例如：If you had taken my advice, you wouldn’t have failed in the examination.
3、表示与将来事实相反的假设时,条件从句和主句所用的谓语动词 与“表示与现在事实相反的假设”的谓语动词相同,或者条件从句中用“were to (should)+动词原形”,
例如：If it were to rain tomorrow, the meeting would be put off.
4. 当条件状语从句表示的行为和主句表示的行为所发生的时间不一致时,动词的形式要依据它所表示的时间来调整.
例如：If you had worked hard, you would be very tired.
5.以上句型可以转换成下列形式：
1）省略if用“were, had, should+主句”.
例如：Were I in school again, I would work harder.
Had you been here earlier, you would have seen him.
但在虚拟条件状语从句中,省略连词的倒装形式的句首不能用动词的缩略形式,如我们可以说：Were it not for the expense, I would go to Italy. 但不能说：Weren’t it for the expense, I would go to Italy.
2）用介词短语代替条件状语从句.
例如：Without air, there would be no living things.
二、虚拟语气用于宾语从句
1.“wish+宾语从句”表示不能实现的愿望,汉语可译为“可惜……”,“……就好了”,“悔不该……”,“但愿……”等.
表示现在不能实现的愿望,从句的谓语动词过去式;
表示将来不能实现的愿望,用“would(could)+动词原形”;
表示过去不能实现的愿望时,用“had+过去分词”或“（could） would+have+过去分词”.
例如:I wish it were spring all the year round.
I wish you could go with us.
We wish we had arrived there two hours earlier.
2.虚拟语气在动词arrange, command, demand, desire, insist, order, propose, request, require, suggest等后面的宾语从句中用“(should)+动词原形”.
例如：We suggest that we (should) have a meeting.
We insist that they (should) go with us.
The doctor ordered that she (should) stay in bed for a few days.
注意：只有当insist作“坚持（认为）”,“坚持（应该）”,suggest作“建议”解时,从句的动作发生在谓语之后,宾语从句才用虚拟语气.当insist作“力言”,“强调”,suggest 作“暗示,表明”解时,宾语从句通常不用虚拟语气.
例如：Mike insisted that he had never stolen anything.
The expression on his face suggested that he was very angry.
三、虚拟语气用于表语从句、同位语从句
作advice, idea, order, demand, plan, proposal, suggestion, request等名词的表语从句和同位语从句,其谓语动词一般要用虚拟语气的结构“（should）+动词原形.”
例如：We all agreed to his suggestion that we (should) go to Beijing for sightseeing.
My idea is that we (should) do exercises first.
四、虚拟语气用于主语从句
在主语从句中,谓语动词的虚拟语气结构用“should+动词原形”的结构,表示惊奇,不相信,惋惜,理应如此等.
例如：It is necessary (important, natural, strange, etc) that we should clean the room every day.
It was a pity ( a shame, no wonder, etc) that you should be so careless.
It will be desired (suggested, decided, ordered, requested, proposed, etc.) that she should finish her homework this afternoon
五、虚拟语气在as if (as though), even if (even though)等引导的表语从句或状语从句中的应用.
如果从句表示的动作发生在过去,用过去完成时；指现在状况,则用过去时,指将来则用过去将来时.
例如：He looked as if he were an artist.
He speaks English so fluently as if he had studied English in England.
Even if he were here, he could not solve the problem.
在上述三种主语从句中,should应意为“应该”,“竟然”,可以省去,但不可换用would,主句所用动词的时态不限.注意：这种从句表示的是事实.如果说话人对这种事实表现出惊奇的情感,就可用虚拟语气.反之,如果不表示惊奇等情感,that 从句也可陈述语气,
例如：It is strange that he did not come yesterday.
It is a pity that you can’t swim.
六、虚拟语气用于定语从句
这种从句常用在It is (high) time (that)…句型中,定语从句的谓语动词用过去式,或should+动词原形（should不能省略,be 用were)来表示,意为“（现在）该……”.
例如：It is (high) time we left (should leave).
It is high time we were going.
七、虚拟语气用于if only 引导的感叹句中
If only I had taken his advice. 我要是听他的话就好了.
If only I were a bird.我如果是一只鸟就好了.
八、虚拟语气在简单句中的应用
1）情态动词的过去式用于现在进行时,表示说话人谦虚,客气,有礼貌或语气委婉,常出现在日常生活中,
例如：Would you be kind enough to open the door?
2）用于一些习惯表达法中,
例如：Would you like a cup of tea?
I would rather not tell you.
You had better go now.
3）用“may+动词原形”表示“祝愿”.may 必须置于句首,
例如：May you be happy!
May you succeed!
