I go to Tokyo
 

Is it correct to say

I go to Tokyo.
watashi wa Toukyou e ikimasu.
わたしはとうきょうへいきます。

I would like to mention the subject in my sentence..

I think instead of the e should be ni. So: watashi wa Toukyou ni ikimasu.
I'm not an expert though and I'm probably wrong, but I try.

What if I mentioned the subject in all the statements that include a subject to make it simple for me and for those who I will teach Japanese in the future?

The sentence.
I understand Japanese.
I am going to mention the subject so can I say?

watashi wa Nihongo ga wakarimasu.
わたしはにほんごがわかります。

Do you understand Japanese?
I know that I can say.
Nihongo ga wakarimasuka.
にほんごがわかりますか。

But, if I wanted to mention the subject (You) Can I just say?
anata wa Nihongo ga wakarimasuka.
あなたはにほんごがわかりますか。

Again I am not sure but ga is topic matter. From my Japanese lessons with my former tutor I think it should be o instead of ga. I know I argued about o instead of ga like crazy. I don't know if that is correct though there are many things I am still confused about and this is definitely one of those. But I do think it's:

watashi wa Nihongo o wakarimasu

and

Anata wa Nihongo o wakarimasu ka?

If I am wrong I am wrong. I still try.

In this case に and へ are interchangeable.

Cool. Thanks!

So if I mentioned the subject here i should use "ni" instead of "e"?
And if I mentioned the subject in an object sentence I should use "o" instead of "ga"..

OK what about this?

Where did you go yesterday?
Can I say
anata wa kinoo doko e ikimashitaka.
あなたはきのおどこえいきましたか。

Or?
anata wa kinoo doko ni ikimashitaka.
あなたはきのおどこにいきましたか。

To keep things simple, both of these sentences are correct. However, you don't need a subject for most of these sentences. あなた would not be used in this sentence as it should be obvious who the speaker is talking to. Also きのう doesn't have an お in it. The particle "e" is written へ.

But why this site state it as "kinoo" "yesterday" so it is きのお
Shouldn't it be "kinou" then? in order to be きのう
Vocabulary - Lesson 4 - Language - Kids Web Japan - Web Japan

And which one of my sentences here is the correct one?
Did you go to Tokyo?
I know that I can say:
Toukyou e ikimashitaka.
とうきょうへいきましたか。

But again, can i mention the subject and say?
anata wa Toukyou e ikimashitaka.
あなたはとうきょうへいきましたか。

Or "ni" instead of "e"?
あなたはとうきょうにいきましたか。