我到日本去旅行了(Past Tense)

It's correct in Taiwan. 

= I had been to Japan last year.

我的朋友在住在日本，所以訪問了她. 我坐飛機從美國到日本青森去了. 我在日本日本待了七個天了.

We will describe the place from the big space to the small space in Chinese.

CA , U.S.A. = "美國加州"

我覺得夏季去最好 , 因為沒有雪. 我不喜歡雪.

我在日本看到了Nebuta, 吃了日本菜, 還和跟我的好朋友炭索探索這個國家.

"和" and "跟" are the same meaning of "with"

"還和我的好朋友"  = and ....with my good friend.

"探索" = explore

"探索這個國家" = explore this country.

我覺得你們應該去旅行日本旅行, 因為 日本很美.

"去日本旅行" = take a trip to Japan

我重要的旅行日本之旅.

"我重要的日本之旅" = My important trip of Japan"

A lot of grammar rules are inverse in Chinese.

No, I think your opinion is not all correct.

There are many people speak Hô-ló-uē in Fujian, they still speak standard Chinese.

We also have many people speak Hak-kâ-ngî , and many Taiwan Indigenous peoples.

They all don't speak Hô-ló-uē.

Someone mentioned the main basis of Taiwanese Mandarin is the Nanjing standard , it's right.

And the generation of my grandfather and my grandmother speak Japanese.

Japanese also have some influences in the pronunciation.

The most important factor is that people are taught the different pronunciation system in Taiwan.

We are taught Mandarin Phonetic Symbols which is not used in Mainland China from 1958.

So it's like the different between British English and American English.

If you using the Hànyǔ Pīnyīn, you will speak like standard Chinese in Mainland China.

海豹跟海獅,一樣不一樣?

"它" = nonhuman subject.  

"它們" = nonhuman subjects. You can use "他們", only if they are male. 

海豹沒有耳朵,反而有兩個小洞.海獅都有耳朵.海狗也有耳朵,所以他到底是海獅它和海豹有區別!
"反而" = but . 

"也" = too.

"海獅有耳朵，海狗也有耳朵" = Sea lion have ears and Fur seal have ears too.

"海獅跟海狗都有耳朵" = Both Sea lion and Fur seal have ears.

"都有" = Both

"所以它和海豹有區別!" =So it is different to Earless seal.

"區別" = "different".

海獅的後面鰭狀肢比海豹的長,可是海豹的前面的鰭狀肢爪子最長.海獅和海豹在土地走起來不一樣，因為它他們的鰭狀肢尺寸不一樣.海豹需要在肚子下匍匐，(它)跟毛蟲很像.海獅在土地可以用和鰭狀肢跑,也可以在動物園跳舞.
"用" = using something. You use the word "with" in English.

"和" = together in the same place. You also use the word "with" in English.

海豹有最多毛,可是我覺得海獅比海豹可愛。因為大部分的海豹的叫聲很响亮，而且很肥.可是我曾經在冰島看過得到海豹.他們在石頭上睡覺著了,看起來真漂亮.

"而且很肥"= It is very fat.

"我曾經在冰島看過海豹"=  I have seen Earless seal in Iceland.

"睡著" = slept