5."Tôi chỉ nói đùa thôi!"

Bạn viết rất tốt :)

g5."Tôi chỉ nói đùa thôi!"

I have to say, Vietnamese words are quite similar to Japanese words in term of meaning, however, when it comes to grammar, I think it's more close to English, so when you try to say something, try to think in English as well to see if that makes sense. "Khi" = when =>

 Khi tôi không biết tiếng Việt đâu = When I don't know Vietnamese. First of all, you do know Vietnamese, even just a little is still consider "know", second of all. how does "when I don't know Vietnamese, my friend said....." make any sense, that's why I said try to think in English as well. What you were trying to say is in fact "Because I don't know much VNese, my friend (took advantage of that and) said...." which is equivalent to my sentence above.

Tôi đã tìn tưởng bạn tôi... 

Tôi đã <font style="background-color: rgb(255, 255, 0);">tin</font> bạn tôi. It's not necessary to say "tưởng". When you say "tin tuong" , that means you say "believe in", when you say "tin" only, that means you say " believe". Even in English, it's not common to say I believe in what you say, it's just "I believe what you say". The impact is different.

Tôi đã nghĩ tôi nên nói "xin chào" cho người Việt ngưng càng tôi nói "xin chào" càng mọi người cười.

Tôi <font style="background-color: rgb(255, 255, 0);">tưởng</font><font style="background-color: rgb(255, 255, 0);"> tôi nói</font> "xin chào" <font style="background-color: rgb(255, 255, 0);">thật</font>, <font style="background-color: rgb(255, 255, 0);">nhưng</font> <font style="background-color: rgb(255, 255, 0);">tôi càng nói mọi người càng cười</font>.

It's really uncommon to say "đã nghĩ", the word "<font style="background-color: inherit;">tưởng" can be consider "thought"</font>

Tôi đã nghĩ người Việt có lẽ "shy"...

Tôi <font style="background-color: rgb(255, 255, 0);">tưởng</font> họ<font style="background-color: rgb(255, 255, 0);"> xấu hổ</font> (xau ho =hazukashii). However, I don't understand why you thought they were shy while in fact they were laughing.

Ngày hôm sau tôi biết "em yêu anh" không phải là "ohayou".

Ngày hôm sau tôi<font style="background-color: rgb(255, 255, 0);"> mới</font> biết "em yêu anh" không phải là "ohayou".

"moi" can be consider it was not utill....that/ only when......do/did/will I........ Basically, my sentence means "it was not until the following day that I realized...."

Tôi đã thật xấu hổ nhưng đùa của bạn tôi rất hay! :p

Tôi <font style="background-color: rgb(255, 255, 0);">cảm thấy rất xấu hổ</font> nhưng trò đùa của bạn tôi rất hay 

 "hazukashii" is adj, you can't treat it like a "verb" right. so you need to add a verb there, which is "feel". Also, it's true that  you add "đã" before a verb to form the past tense of that verb, however,  in many cases, it's unnecessary to do so either because the context already indicates that you're talking about the past or because of some specific verbs and it will just make what you say sound unnatural. Seriously, it's really hard to show you all this, you just need to know that, for example, there are some exceptions in the way of reading kanji, like the word "megane", all I can do is just simply know that, I can't really expect to know why it's like that.

5."Tôi chỉ nói đùa thôi!"