肯定句
主词
to have
过去分词
She
has
visited.
否定句
主词
to have + not
过去分词
She
has not (hasn't)
visited.
疑问句
to have
主词
过去分词
Has
she
visited?
否定型疑问句
to have + not
主词
过去分词
Hasn't
she
visited?
现在完成时，以 to walk 为例
肯定句
否定句
疑问句
I have walked.
I haven't walked.
Have I walked?
You have walked.
You haven't walked.
Have you walked?
He, she, it has walked.
He, she, hasn't walked.
Has he, she, it walked?
We have walked.
We haven't walked.
Have we walked?
You have walked.
You haven't walked.
Have you walked?
They have walked.
They haven't walked.
Have they walked?
现在完成时的作用
现在完成时是用来表明现在与过去的连结。动作的时间是在过去，并没有一个具体的时间点。我们往往更强调动作的结果，而非动作本身。
要留意！也许您的母语中也有相似的动词时态，但意思不见得是一样的。
现在完成时是用来描述：
始於过去的动作或状态，到目前仍持续着。I have lived in Bristol since 1984 (= 而且到现在还是。)
在一段时间内做过丶但还没有结束的动作。She has been to the cinema twice this week (= 而且这周还没过完。)
在过去到现在之间的一段不明确的时间内重复发生的动作。We have visited Portugal several times.
在不久前完成的动作，以「just」来表达。I have just finished my work.
发生时间并不重要的动作。He has read 'War and Peace'. (= 阅读的结果很重要)
注意：如果想要针对何时丶何地丶何人给予或询问细节，我们会用过去简单时。关於这方面可以参考这篇文章，了解如何在现在完成时和一般过去时之间做选择。
始於过去且目前持续在做的动作
They haven't lived here for years.
She has worked in the bank for five years.
We have had the same car for ten years.
Have you played the piano since you were a child?
当提及的时间点还没结束
I have worked hard this week.
It has rained a lot this year.
We haven't seen her today.
在过去到现在之间不具体的时间范围内重复做的动作
They have seen that film six times
It has happened several times already.
She has visited them frequently.
We have eaten at that restaurant many times.
刚结束的动作（加 just）
Have you just finished work?
I have just eaten.
We have just seen her.
Has he just left?
当动作的确切时间并不重要或未知时
Someone has eaten my soup!
Have you seen 'Gone with the Wind'?
She's studied Japanese, Russian, and English.
进一步了解如何在现在完成时中使用 ever丶never丶already 和 yet，以及在现在完成时中使用 for 和 since。