怎么理解 现在完成时 的这种用法 表示过去发生或已经完成的动作对现在造成的影响或结果

其实要理解语法还是用人家本国的语言理解比较好,很简单很明了,汉语有太多歧义的地方,并且英文的解释一般很好理解,并没有难懂的单词.
定义：We use the present perfect tense to talk about things that started in the past and still have some connection with the present.（其实就是上面说的那个意思）
用法：1.Actions which started in the past and are still continuing
eg：He has lived in Canada for five years.
(He started living in Canada five years ago,and he's still living there now.)
2.Actions which happened at some unknown time in the past
eg：I've already seen that film.I don't want to see it again.
(It doesn't matter when I saw it.)
3.Actions which happened in the past,but have an effect in the present
eg：I've lost my keys.\x05
（I don't have the keys.They are still missing.）