Hello Jake, great work:)

When: 當...的時候 or 當...時 

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大家可以幫助我嗎？謝謝！

My focus with this one was expressing reason and the when+ verb pattern.

我想說一下關於我喜歡學習普通話什麼 [的原因]。- I want to talk a bit about what I like about studying mandarin.

我總是喜歡學習新的東西。- i have always liked studying new things
所以我度 [讀] 過很多的有意思的書。- So I've read a lot of interesting books.
可是我最喜歡的是說外語。- but my favorite thing is speaking foreign language,
再 [當我學會] 說一個外語 [時]，我會頂 [可以了解一個] 新文化。- when I speak a foreign language, I can understand a new culture.
還 [而且]，因為我喜歡學習言語，所以在說外語 [時]，我很高興 [開心]- also， because I like to study language, when I am speaking a foreign language I am very happy,

大家可以幫助我嗎？謝謝！

My focus with this one was expressing reason and the when+ verb pattern.

我想說一下關於我喜歡學習普通話的原因什麼。- I want to talk a bit about what I like about studying mandarin.

我總是喜歡學習新的東西。- i have always liked studying new things
所以我度看過很多的有意思的書。- So I've read a lot of interesting books.
可是我最喜歡的是說外語。- but my favorite thing is speaking foreign language,
再說一個外語，我會頂新了解新的文化。- when I speak a foreign language, I can understand a new culture.
還有，因為我喜歡學習言語，所以在說外語，我很高興- also， because I like to study language, when I am speaking a foreign language I am very happy,

大家可以幫助我嗎？謝謝！

My focus with this one was expressing reason and the when+ verb pattern.

我想說一下關於我喜歡學習普通話什麼。- I want to talk a bit about what I like about studying mandarin.

我總是喜歡學習新的東西。- i have always liked studying new things
所以我度读過很多的有意思的書。- So I've read a lot of interesting books.
可是我最喜歡的是說外語。- but my favorite thing is speaking foreign language,
再說一個外語，我會頂新文化。当我会说一门外语时，我就能理解新的文化-（when:当......时） when I speak a foreign language, I can understand a new culture.
還，因為我喜歡學習言語，所以在說外語，我很高興。还因为我喜欢学习语言，当我讲外语时我会很开心（language+。- also， because I like to study language, when I am speaking a foreign language I am very happy,

大家可以幫助我嗎？謝謝！

My focus with this one was expressing reason and the when+ verb pattern. Your purposes: 表达理由和When+动词时态

我想說一下關於我喜歡學習普通話的事情。- I want to talk a bit about what I like about studying mandarin.

我總是喜歡學習新的東西。- I always like studying new things
所以我度過很多的有意思的書。- So I've read a lot of interesting books.
可是我最喜歡的是說外語。- but my favorite thing is speaking foreign language,
每当学会说一门外语，我都能了解一种新的文化。- when I speak a foreign language, I can understand a new culture.
另外，因為我喜歡學習语言，所以我很开心能够说一门外语- also， because I like to study language, when I am speaking a foreign language I am very happy,