A run-on sentence joins two complete sentences with nothing between them. A comma splice joins them with only a comma. To fix either one, use a period, a semicolon, a comma with and, but or so, or a word such as because. Six rules and 40 fix-it questions below show you how.
40 questions8 levels of 56 rulesLevel 1 free
1 in 10
What we measured
of the 8,000 most recent writing responses we evaluated were told to fix a run-on sentence or comma splice.
How do you fix a run-on sentence?
The comma test: could the words after it stand alone?
Stop at each comma and ask: could the words after it stand alone as a sentence? If yes, check the words before it too. If both sides could stand alone, the comma is the mistake, and you have a comma splice. With no punctuation at all between them, you have a run-on. If one side cannot stand alone, as in "After lunch, I called her", the comma is fine. It is also fine with and, but or so right after it.
Right:When the rain stopped, we walked home.
Right:The cafe was full, so we sat outside.
Not:The cafe was full, we sat outside.Instead:The cafe was full, so we sat outside.
Not:My exam is on Friday I need to study.Instead:My exam is on Friday, so I need to study.
Four ways to fix a comma splice
Take this splice: "My roommate was sick, I brought her soup." You have four fixes. Use a period: My roommate was sick. I brought her soup. Use a semicolon when the ideas are closely linked: My roommate was sick; I brought her soup. Keep the comma and add and, but or so: My roommate was sick, so I brought her soup. Or use because or although: Because my roommate was sick, I brought her soup.
Right:The room was cold; I kept my coat on.
Right:I stayed until the end because the talk was interesting.
Right:Although the talk was long, I stayed until the end.
Not:The room was cold, I kept my coat on.Instead:The room was cold, so I kept my coat on.
Not:Although it was late, but we kept working.Instead:Although it was late, we kept working.
However and therefore need a period or a semicolon
However and therefore look like joining words, but they cannot join two sentences with a comma. Put a period or a semicolon in front of them and a comma after them. The same goes for as a result and for example. To keep the comma, change the word: but for however, so for therefore. Inside one sentence, commas around however are fine: "Most of us, however, passed."
Right:The line was long; however, it moved quickly.
Right:Parking is expensive. Therefore, I take the bus.
Right:The food, however, was excellent.
Not:The line was long, however, it moved quickly.Instead:The line was long; however, it moved quickly.
Not:Parking is expensive, therefore I take the bus.Instead:Parking is expensive, so I take the bus.
A complete sentence needs a subject and a verb
A sentence needs a subject (who or what it is about) and a verb (what that person or thing does or is). Forms such as studying, being and to study cannot be the main verb alone, so "My cousin studying law" is unfinished. Long descriptions fool students too. "The student who sits next to me" has seven words, but the student never does anything, so it is still not a sentence.
Right:My cousin is studying law in Boston.
Right:The student who sits next to me speaks three languages.
Not:My cousin studying law in Boston.Instead:My cousin is studying law in Boston.
Not:The student who sits next to me in class.Instead:The student who sits next to me in class speaks three languages.
Fragments often start with Because, Which or For example
A fragment is a piece of a sentence with a period after it. Because and which start parts that lean on another sentence, so "Because I was sick." is unfinished. You can start with Because when a second half follows: Because I was sick, I stayed home. Attach a which part to the sentence before it with a comma. After For example, write a full sentence with its own subject and verb.
Right:I stayed home because I was sick.
Right:The bus was late, which annoyed everyone.
Right:Some dorms have quiet hours. For example, my dorm is silent after ten.
Not:I stayed home. Because I was sick.Instead:I stayed home because I was sick.
Not:The bus was late. Which annoyed everyone.Instead:The bus was late, which annoyed everyone.
Not:Some dorms have quiet hours. For example, my dorm after ten.Instead:Some dorms have quiet hours. For example, my dorm is silent after ten.
Watch for a missing is, are, it or there
Two small gaps cause many of these errors. The first is a missing is or are: "This class very difficult" has no verb, so write "This class is very difficult." The second is a missing subject. English does not allow "Is important to sleep." Use it as an empty subject: "It is important to sleep." Use there to say that something exists: "There are three cafes on campus."
Right:This class is very difficult.
Right:It is important to sleep well before an exam.
Right:There are three cafes on campus.
Not:This class very difficult.Instead:This class is very difficult.
Not:Is important to sleep well.Instead:It is important to sleep well.
Not:Have three cafes on campus.Instead:There are three cafes on campus.
Where this costs you marks on the TOEFL
The TOEFL has no grammar section, so run-ons cost you marks inside Writing and Speaking. In Write an Email and the Academic Discussion you type your own sentences at speed, and that is when commas start doing the job of periods. One comma splice is a small slip. A chain of them is different: the reader has to stop and work out where one idea ends. In the Interview nobody hears a comma, but the same habit appears as one long answer held together by and, and, and. Finish each idea, pause, then start the next. That pause is a spoken period.
If your first language is...
First language
What to watch for
Chinese speakers
A Chinese sentence can hold several complete thoughts separated by commas, with the period arriving only when the whole idea is finished. English does not work that way. Each complete thought needs its own period or a joining word, so check every comma in your longer sentences.
Russian and German speakers
Your languages allow two complete sentences joined by a comma alone, so "The exam was hard, we were tired" looks normal. Because it is correct at home, it will not look wrong to you in English. Test each comma on purpose, and add a joining word or use a period.
Spanish, Portuguese, Italian, Arabic and Russian speakers
Spanish, Portuguese and Italian usually drop the subject, so "Is important to sleep" feels complete. Arabic and Russian usually leave out is and are in the present tense, so "This class very difficult" feels complete. An English statement needs both a subject and a verb.
Hindi, Urdu, Chinese and Vietnamese speakers
These languages often pair a word for although with a word for but in one sentence, as Hindi does with halanki and lekin. English uses only one. Write "Although it was late, we kept working" or "It was late, but we kept working", never both.
More tips from Anil and Kailash
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All 560 questions across 14 skills
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Speed rounds and a review list of your own mistakes
Try these before you open an answer. Each one explains why, not only what.
My phone died_____ I could not call you. (, / no punctuation / , so)Show the answer
, so
Both sides could stand alone as sentences, so a comma by itself is not enough, and no punctuation is worse. A joining word such as so holds them together.
_____ class, I went to the gym. (We had / After / I left)Show the answer
After
After class cannot stand alone, so the comma is fine. The other choices put a full sentence on both sides of the comma.
Find the mistake: The bus was late, I walked to class this morning.Show the answer
The bus was late, so I walked to class this morning.
Both sides of the comma could stand alone, so the comma is the mistake. Use a period, or keep the comma and add so.
I was tired, _____ I went to bed early.Show the answer
so
Both sides are full sentences, so the comma needs a joining word. So shows the result. And would also work.
Put in order, with one piece left over: It was raining, [I / however / took / so / the / bus].Show the answer
It was raining, so I took the bus.
There is a full sentence on each side of the comma, so it needs a joining word. So can do that job after a comma. However cannot.
A run-on sentence is two or more complete sentences joined with no correct punctuation between them, as in "The lecture ended we went home." Length has nothing to do with it: that example has only six words. Many teachers also use run-on as a general name that includes comma splices. Fix one with a period, a semicolon or a joining word.
What is a comma splice?
A comma splice is two complete sentences joined by only a comma, as in "The lecture ended, we went home." Each side could stand alone, and a comma is not strong enough to hold two sentences together. Replace it with a period or a semicolon, or keep it and add and, but or so after it.
Is a long sentence always a run-on?
No. A sentence can run to forty words and be perfectly correct if its parts are joined properly, and a five-word sentence can be a run-on. Structure is what matters. Count the complete sentences inside it and check that each join has a period, a semicolon or a joining word. A long sentence may be hard to read, but that is a style problem.
What is a sentence fragment?
A fragment has a capital letter and a period but is not a complete sentence, as in "Because the bus was late." The reader is still waiting to hear what happened. You may start a sentence with because, as long as a second half follows: "Because the bus was late, I missed the quiz." Some fragments have no verb at all.
Do run-on sentences lower my TOEFL score?
One comma splice is only a small slip. A repeated pattern matters more, since it makes your writing harder to follow. In our data, one in ten recent writing responses were told to fix one. A quick comma check at the end of each response is an easy way to protect your Writing band.
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