present continuous用法（中文解釋）急！！

Present continuous tense主要有兩個功用
1.現在進行的
a) Look! The bridge is falling down! 看!那條橋正在掉下來!
這句話的用法就是句子 發生在說話的時候

b) I am studying in primary 4. 我正在就讀小學四年級.
這句話的用法就是在 現階段,行動正在進行

2.將來會發生的(你很肯定還有你已經計劃好了)
用你的例子: Uncle Tim is flying back to Canada this week 添叔叔會在這個星期回加拿大去.
就是說,添叔叔已經買了機票,或收拾好行李,很肯定會回去加拿大.

至於為什麼要用is+verb+ing我想是不能解釋的
不過我想提提你
I 用 am eg. I am reading books.
You/We/They 用 are eg. We are playing football.
He/ She/It 用 is eg.She is running.

present continuous tense how to use????
present continuous tense 是正在進行中，還未完成的 action.
Example:
I am watching TV
I am studying for my exam.
I am looking for my glasses.
He likes making up story.

如果你是指 Present continuous tense 的話，就是現在進行式。即是現在還在做緊的動作，
例如：我講緊電話。I am answering the phone. ∕　我想緊如何解決這難題。I am thinking how to solve the problem.

present continuous(禮物連續 )