Actually, I have to say you really neednot bother that, except you are determined to be Chinese professor, :) The commone people even don't know the precise differences. If your Chinese has been excellent and professional level. However, I should give your earnest spirit a like:) As a teacher I recommend you know them in a phrase, that is combining with another word or words to understand the usage of these two words , and to gradually feel diferent usages between them. :)

你要是有兴趣我可以继续发，要是没有就算了，呵呵。高中语法。

虚拟语气是一种假设的用法。一种假设是可以发生的叫“真实条件句”，如If he doesn’t hurry up he will miss bus. 假设的事情会发生。令一种假设是不会发生的叫“非真实条件句”，虚拟语气就是非真实条件句。我们前面已经学过，就是情态动词加完成时的用法也属于虚拟语气一种。 我们主要考的就是在虚拟语气中谓语动词的形式，用法。特别注意：虚拟语气和时态无关。 A. 对过去情况的虚拟 条件从句的谓语动词 主句谓语动词 had done Would / should / might / could have done 过去完成时形式 Would等词+现在完成时形式 形式： 例题1. If I ____ you would drop in, I would have stayed home waiting for you. A. have known B. knew C. would know D. had known 解析，本题看着好像是考时态，（虚拟语气的题看起来好像都是在考时态），但注意本句的假设是“非真实条件句”（如果我早知道你来），事实是我之前不知道，所以这句是虚拟语气用法。套用上面的公式，主句用的是would have done，If引起的，表示条件的从句，用had done，表示对过去的假设用相当于过去完成时的形式，注意：和时态无关。选D. 例题2. —If you ____ to my birthday party last Saturday, you would have met our old friend Jim. —Oh, what a pity! A. came B. had come C. would come D. would have come 解析，从句意看，如果你上周六来我参加我的聚会，说明上周六没来，属于对于过去的假设，又因为是在If从句上，用过去完成时形式。选B. 例题3. If you’d written earlier, I’d have known when you ____ to go on holiday. A. want B. have wanted C. wanted D. will want 解析，本句句意，如果你早就给我写信，当你想要度假的时候我也就早就知道了。说明之前没给我写信，而且我也不知道，与过去情况相反。注意，这题when后面的从句不是虚拟语气，它不是假设的句子，那么我们也就可以通过前面对过去情况的假设来判断，when后面的从句是一般过去时。选C.

有时候常来玩儿.----No, I don't think we say like that in general, We say 有时间常来玩, which means come over here to have fun if you have time. Or the precise time, such as 你能告诉我具体时间么，Can you show me accurate time? 你买衣服了多长时间了-----你买这件衣服多久了。 Which means how long you kept this skirt. 时候，Ok, I make a sentence, 我小的时候， When I was young. 他在我最困难的时候帮助了我。--He favored me when I was in tough time. Got that. I recommend you not just recite the only word's meaning, combine it with another word or make a whole sentence, just short is ok, so in might easy for you to get hold of them when you learn another language. Or in the form of phrase,

It might be in the form of post-() help such as post-operative help. The post is a suffix meaning after sth. hope I am right.