A complete idea has a subject and a verb and makes sense by itself. When two complete ideas sit together, something stronger than a comma must separate them. This skill appears in every extended writing task in spelling, punctuation and proofreading.
What can separate two complete ideas?
There are four reliable options. Learn them as a short menu.
- A full stop and a capital letter: The bus was late. We missed the start.
- A semicolon: The bus was late; we missed the start.
- A comma plus a joining word (and, but, so, yet, or): The bus was late, so we missed the start.
- A joining word that makes one idea depend on the other (because, although, when): We missed the start because the bus was late.
A comma on its own is never on the menu between two complete ideas.
How do you test a comma?
Cover the words after the comma. Does the first part stand alone as a sentence?
Now cover the words before the comma. Does the second part stand alone? If both do, the comma is too weak.
Worked example
Correct this paragraph.
The library closes at five, we have only an hour. Nadia wanted a novel, Ben needed a science book. The shelves were crowded however they found both.
Step 1. “The library closes at five” is complete. “we have only an hour” is complete. Only a comma joins them. Use “so”: The library closes at five, so we have only an hour.
Step 2. “Nadia wanted a novel” and “Ben needed a science book” are complete and closely related. A semicolon fits: Nadia wanted a novel; Ben needed a science book.
Step 3. “however” is joining two complete ideas with no punctuation at all. Use a semicolon before it and a comma after it: The shelves were crowded; however, they found both.
Corrected:
The library closes at five, so we have only an hour. Nadia wanted a novel; Ben needed a science book. The shelves were crowded; however, they found both.
The mistake to watch for
A common slip is to treat “however”, “therefore” and “also” as if they were joining words like “and”.
Mistaken: I revised all week, therefore I felt ready.
“Therefore” does not join the two ideas, so the comma still splices them. Correct versions: “I revised all week, so I felt ready.” or “I revised all week; therefore, I felt ready.”
Check yourself
Fix the punctuation in each sentence.
1. The museum was quiet, we could hear our own footsteps.
Show answer
Two complete ideas joined by a comma. Options: “The museum was quiet. We could hear our own footsteps.” or “The museum was quiet; we could hear our own footsteps.” or “The museum was quiet, so we could hear our own footsteps.”
2. Amir trained daily, however he did not make the team.
Show answer
Use a semicolon and a comma: Amir trained daily; however, he did not make the team. Or use “but”: Amir trained daily, but he did not make the team.
3. Which one is correct? (a) She was tired, she kept working. (b) She was tired, yet she kept working.
Show answer
(b). The comma plus “yet” correctly joins the two complete ideas. Sentence (a) is a comma splice.
Where this leads next
Once your sentences are separated correctly, learn to tell a careless slip from a real gap: distinguish a typing error from a knowledge gap. For the grammar behind joining words, see sentence building and paraphrase.
A teacher can hear where you pause when you read your draft aloud, which is how punctuation is placed most naturally. This is part of what we do in online one-to-one English as a Second Language tuition.