A：這句犯了一個相當常見且非常嚴重的錯誤，也就是英文所謂的「逗點謬誤」 (comma fault or run-on error)，即兩個句子之間沒有連接詞而是用逗點來當連接詞。這種句子被稱為 run-on sentences。問題的癥結在於 therefore 是個副詞，卻被當作連接詞。所以問題中的句子應改為
- The dollar has gone down against the yen, and therefore Japanese goods are more expensive for Americans. (美元兌日元的匯率下跌了，因此日本商品對美國人來說比較貴了) － 這句沒有下句好。
或
- The dollar has gone down against the yen; therefore, Japanese goods are more expensive for Americans.
上句係用分號來代替「, and」。除 and 外，分號亦可用來取代 but 和 or 等對等連接詞；分號後面的字須小寫。例如：
- The president did not approve the military operation; he suggested several options. (總統不同意這項軍事行動；他建議幾個選擇方案) － 在這句中，分號取代了「, but」。
在英文中，為了承接上一句的意思，我們往往在新的句子開頭使用 however, therefore, consequently, nonetheless, nevertheless 或 as a result 等轉折詞 (transitional phrases)。這些轉折詞又叫做「連接副詞」(conjunctive adverbs, connectives)，亦即連接兩件對比、差異的事情或兩個對比、不同的觀念所用的副詞 (並非連接詞)。
轉折詞有如上下兩句的橋樑，可使兩個句子之間的轉折比較平順。它們都位在下一句的開頭，其後通常有逗點，但前面絕不可有逗點，否則就會發生上述錯把連接副詞當連接詞的逗點謬誤情況，其中又以 however 最常發生這樣的錯誤。正如上述，我們亦可使用分號來使上下兩句更加無縫接軌。例如：
- I am leaving now, however, I will be back on Saturday to attend your wedding ceremony. (我現在要離開了，但我禮拜六會回來參加你們的婚禮) (誤)
- I am leaving now. However, I will be back on Saturday to attend your wedding ceremony. (正)
- I am leaving now; however, I will be back on Saturday to attend your wedding ceremony. (正)
- We talked until the early hours, and consequently I overslept. (我們聊到凌晨，結果我睡過頭了) (正) (不佳)
- We talked until the early hours. Consequently, I overslept. (正)
- We talked until the early hours; consequently, I overslept. (正)
- John was ill, as a result, he could not come. (約翰病了，所以他不能來) (誤)
- John was ill. As a result, he could not come. (正)
- John was ill; as a result, he could not come. (正)
For example, of course 和 on the contrary 等片語也有轉折詞的功能。例如：
- Business is booming; for example, Peter has made one million dollars since April last year. (生意欣欣向榮；譬如說，彼得自去年四月以來已賺了一百萬美元) (正)
- Everyone knows he has a mistress, of course, he will never admit it. (每個人都知道他有小三，當然了，他永遠都不會承認) (誤)
- I did not go to London, on the contrary I went to Paris. (我沒有去倫敦而是去了巴黎) (誤)
另外，so 除了一般常見的副詞和連接詞用法外，還有相當於 therefore 的連接副詞或轉折詞功能。
- Tom came down with a cold. So, he couldn’t come to the party. (湯姆得了感冒。所以他不能來參加派對) (正)
- Tom came down with a cold; so, he couldn’t come to the party. (正)