therefore 是副詞 (adv.)，表示「因此、所以」，用在英文作文幫你的文章加分!
(例) It rained heavily last Sunday. Tom stayed at home all day. (缺轉折語)
*加上therefore會使英文作文更高分!
It rained heavily last Sunday. Therefore, Tom stayed at home all day. (獨立成兩個句子，使用句號)
= It rained heavily last Sunday; therefore, Tom stayed at home all day. (用分號來連接因果兩句)
= It rained heavily last Sunday, and therefore Tom stayed at home all day. (用 and 來連接因果兩句)
注意：
(1) therefore 不是連接詞，所以沒有連接句子的功能，如果要將因果兩句合為一句話，必須使用分號，或是加上連接詞 and。
(2) therefore 放句首或分號後面時，通常需要在其後加上「逗號」，再接句子。
連接詞是什麼？
連接詞（conjunction）是一種詞性，通常被放置於兩個句子中間，用來連接「詞語、短語、句子、段落」等。要留意的是，只有「連接詞」可以用於連接前後句，「副詞」沒有這個用法！
[講解] : 兩個句子寫在一起要用連接詞來連接
**句子, 句子(X) → 句子, 連接詞 句子
1)He went to a restaurant, he ate a lot.(X)
→He went to a restaurant, and he ate a lot
2)It was a beautiful day, the sun shone brightly.(X)
→It was a beautiful day, and the sun shone brightly.
3)I used to play tennis a lot, I don’t play very often now.(X)
→I used to play tennis a lot, but I don’t play very often now.
4)It rained a lot, they stayed at home.(X)
→It rained a lot, so they stayed at home.
= They stayed at home because it rained a lot.
( so 和 because 不能放在同一個句子！)
So 怎麼用？
So 跟 therefore 一樣，是「因此，所以」的意思，然而，so 的詞性是連接詞，所以用法和 therefore (副詞)有些許差異。
It rained a lot, so they stayed at home.
=It rained a lot. Therefore, they stayed at home.
=It rained a lot; therefore, they stayed at home.
=It rained a lot, and therefore they stayed at home.
在以上的例句中，so 連接了前後兩個子句，形成一個完整句。