一個句子若以副詞片語或子句做開頭，那麼該副詞片語或子句的後面應使用逗點來與主要主句 (即獨立子句) 隔開。儘管有人認為可以不用，但使用逗點的主要目的是要讓句子易讀易懂，對於句意的正確傳達有很大的助益。不過，若句子的開頭並非副詞片語或子句，而是一個字的副詞 (如 Now, Today, Here 等)，那麼它們的後面通常不用逗點。例如：
- In the center of Taipei, a lot of people fell victim to pickpockets. (在台北市中心，許多人遭到扒竊) (正)
- On Wednesday 2nd December, Jack broke up with his girlfriend. (12 月 2 日星期三，傑克跟他女友分手) (正)
- On Wednesday 2nd December Jack broke up with his girlfriend. (誤)
- When I was a kid, I would always swim in the sea. (小時候我常常在海中游泳) (正)
- After Jack had broken up with his girlfriend, he suffered from depression. (傑克跟他女友分手後得了憂鬱症) (正)
- Here they disagree with us. (在這一點上，他們和我們看法不同) (正)
- Yesterday Jack saw his ex-girlfriend having dinner with a man in a restaurant. (昨天傑克看見他前女友和一名男子在一家餐廳共進晚餐) (正)
然而，若副詞片語或子句位在句末，則無須使用逗點。例如：
- Jack broke up with his girlfriend on Wednesday 2nd December. (正)
- A lot of people fell victim to pickpockets in the center of Taipei. (正)
- I would always swim in the sea when I was a kid. (正)
- Jack suffered from depression after he had broken up with his girlfriend. (正)
副詞片語亦可能位在句子中間，此時它們前後都可使用逗點，但通常不用。例如：
- I’m not my parents’ biological son. My father, on his deathbed, told me the truth. (我不是我父母親生的兒子。我父親臨終時告訴了我真相) (正)
- I’m not my parents’ biological son. My father on his deathbed told me the truth. (正)
值得注意的是，若句子開頭是修飾主要子句主詞的分詞片語，那麼其後一定要有逗點。例如：
- Having read our petition, the principal has decided to meet with us. (看了我們的陳情書之後，校長已決定和我們會晤) (正)
- Having read our petition the principal has decided to meet with us. (誤)
- Disappointed almost to the point of tears, I have decided not to talk politics at all from now on. (失望到幾近落淚的程度，我已決定從此完全不談政治) (正)
- Disappointed almost to the point of tears I have decided not to talk politics at all from now on. (誤)