Present Perfect in Genki I


In the Genki I textbook, they seem to reference the present perfect tense
three times, but each with a different grammar. Here they are, with book-style
English translations:

> 日本に行っています。(Ch 7, 〜ている form)  
>  _I have gone to Japan._
>
> 日本にもう行きました。(Ch 9, simple past form with もう)  
>  _I have already gone to Japan._
>
> 日本に行ったことがあります。(Ch 11, 〜ことがある form)  
>  _I have had the experience of going to Japan._

I'm not really sure about the semantic and grammatical differences between the
three, even in English (although the translations may be imperfect). How can I
understand them in relation to each other?

Also, does it ever make sense to combine the forms? Like,

> 日本に **もう** 行っています。  
>  _I have **already** gone to Japan (?)_
>
> 日本に **もう** 行ったことがあります。  
>  _I have **already** had the experience of going to Japan (?)_




> 日本に行っています。  
>  I have gone to Japan.

We don't use this phrase in Japanese. I don't think it corresponds with the
English translation, which may be translated into

> 日本に行きました。

Now there is a big contradiction in this English phrase.  
If you have gone to Japan, you are not here. But you said to me "I have gone
to Japan." Then, where are you now and whom are you talking to?

As a natural Japanese phrase relating to the first one, you can say

> 彼｛かれ｝/彼女｛かのじょ｝は日本に行きました。だから彼｛かれ｝/彼女｛かのじょ｝はここにはいません。  
>  He/She has gone to Japan. So, he/she is not here.

If I hear the phrase "日本に行っています", I image like that you are on a plane flying
to Japan. In this case we say "日本に向｛む｝かっています- We are heading for Japan"
instead.

* * *

> 日本に **もう** 行きました。

Though this sentence has the same contradiction as that of the first one, it
is natural as Japanese.  
You can also say it as " **もう** 日本に行きました" with the same meaning.

* * *

> 日本に行ったことがあります。  
>  I have had the experience of going to Japan.

The third Japanese sentence has no problem at all.  
It could be translated as "I have been to Japan", right?

* * *

> 日本にもう行っています。 I have already gone to Japan (?)  
>  日本にもう行ったことがあります。 I have already had the experience of going to Japan (?)

As you think the former Japanese doesn't make sense, because it has the same
problem as that of the first sentence.  
The latter sentence is a good Japanese.

