這兩個副詞均意為「部分地」(in part)，所以在一些上下文中，不少人認為 partially 和 partly 是可以互換的，如 The stream was partly polluted. (這條溪流遭到部分污染) 和 The stream was partially polluted. 這兩句對大多數英美人士來說意思是完全相同的。然而，事實上，它們的意思和用法並不完全相同。
Partially 意指「不完全地」(not completely; to a limited degree/extent)，是 completely 的反義詞，而 partly 意指「僅涉及一部份，並非全部；有幾分」(not wholly; to some degree/extent)，是 wholly 的反義詞。所以，partially deaf (弱聽) 是指整體的聽力減弱，而 partly deaf 則是指只有一耳聾了；a partially finished report 可能指一份已完全寫好但還需要做最後潤飾的報告，而 a partly finished report 可能指一份有些部分已完成，但其他部分可能都還沒開始撰寫的報告；a partially successful operation 意為「不完全成功的手術」，而 a partly successful operation 意為「有幾分成功的手術」。partially 和 partly 的語意差異在 His book, which was written partly in English and partly in French, was only partially completed when he died. (他的那本書，一部分用英文撰寫，一部分用法文撰寫，到他死時只完成了一部分) 這句中更顯而易見。
此外，partially 和 partly 的用法也不盡相同。partially 在使用上不若 partly 來得寬鬆。partially 須直接放在它所修飾的動詞、形容詞或其他副詞的前面，不可直接用於其他任何詞類之前，而 partly 則可修飾整個片語，如 He is partially responsible for the accident. (他對該事故負有部分責任)；Her father is partially sighted. (她的父親弱視)；The airline is partially owned by British Airways. (這家航空公司部分隸屬於英國航空公司)。所以，在下面三個句子中，我們不能用 partially 來代替 partly：
- Inflation has remained low, partly as a result of cheaper imports. (通貨膨脹率低，部分原因是進口產品價格較低所致)
- I’ll admit I was partly to blame. (我承認我也有錯)
- She was shaking all over, partly because she was so angry with John and partly because she was so afraid. (她全身發抖，一半是因為她對約翰氣憤不已，一半是因為非常害怕)
“And the U.S. stock market dropped yesterday significantly, partially because some investors were concerned that would happen.”
摘自CNN10 03/23/18
你應該看過電視台打錯字幕的畫面吧！？就像我們也會用錯中文一樣，不是每個美國人的英文都非常好，用錯的情況所在多有。正確的用法是 partly because，無庸置疑。