Sentence Boundaries

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In formal, academic contexts, writing complete sentences (also called independent clauses or sentences) is important to ensure your reader understands your writing. A complete sentence must at least contain a subject, a verb, and a complete idea. Four types of sentence structures are illustrated in the table below. In each of the following examples, the independent clause is bolded, and the dependent clause or phrase is italicized. Keep in mind that dependent clauses/phrases can often go before or after the independent clause as long as your wording is logical.

Type	Structure	Example
Simple	1 independent clause	They didn't provide a lot of background information.
Complex	1 independent clause + 1 dependent clause/phrase (option 1) 1 dependent clause/phrase + 1 independent clause (option 2)	They dove right into the methods even though they didn't provide a lot of background information. (option 1) Even though they didn't provide a lot of background information, they dove right into the methods. (option 2)
Compound	1 independent clause + 1 independent clause	They didn't provide a lot of background information, but they dove right into the methods.
Complex-Compound	2 or more independent clauses + 1 or more dependent clauses/phrases	They didn't provide a lot of background information, but they dove right into the methods, providing some more information about the research.

To see rules regarding comma and semicolon usage for each type of structure, please see our Commas and Semicolons chapter.

Problems arise when writers do not include all the required elements to make a complete sentence. Three common problems include comma splices, run-on sentences, and sentence fragments. You have many options to choose from when correcting these mistakes, but which option you choose can change the tone and interpretation of your writing, so make your choice carefully. Regardless of how you revise, make sure the sentence is still grammatical afterward.

Click each topic below to learn more about how to revise common sentence boundary errors

Comma Splices

A comma splice is when two complete sentences are joined with just a comma. Often, people make this kind of error when they feel that the two complete ideas are closely related, but a comma is only a slight pause (both grammatically and verbally) and is not sufficient to fully separate two complete sentences. To correct a comma splice, you have many options.

Let’s revise the following sentence containing a comma splice: She focused on the products that are on sale, she can’t afford them.

Option 1:

Change the comma to a semicolon

Revision: She focused on the products that are on sale; she can’t afford them.
Explanation: A semicolon is a stronger pause than a comma but not as strong as a period. It signals to the reader that the two independent clauses are closely related in some way but not specifically how.
Analysis: Here, a semicolon is not the best choice because the reader needs additional information to understand the relationship between the two clauses.

Option 2:

Change the comma to a period

Revision: She focused on the products that are on sale. She can’t afford them.
Explanation: A period is a strong pause between independent clauses, signaling to the reader that the ideas in the two clauses are not strongly related enough to be in the same sentence.
Analysis: A period is also not the best choice because the reader needs additional information to understand the relationship between the two clauses.

Option 3:

Insert a coordinating conjunction (FANBOYS)

Revision 1: She focused on the products that are on sale, but she can’t afford them.
Revision 2: She focused on the products that are on sale but can’t afford them.
Explanation: A coordinating conjunction signals to the reader that there is a particular relationship between the two clauses or phrases. But indicates contrast.
Analysis: Both revisions are good because the reader can clearly see the relationship between the two clauses. Furthermore, revision 2 is better because deleting the second “she” improves sentence concision.

Option 4:

Use an adverbial conjunction plus punctuation

Revision: She focused on the products that are on sale; however, she can’t afford them.
Explanation: An adverbial conjunction signals to the reader that there is a particular relationship between the two independent clauses. A semicolon precedes the "however," and a comma follows it. However indicates contrast.
Analysis: This is a good revision because the reader can clearly see the relationship between the two clauses.

Option 5:

Use a subordinating conjunction plus punctuation if necessary

Revision: She focused on the products that are on sale even though she can’t afford them.
Explanation: A subordinating conjunction signals to the reader that there is a particular relationship between the two independent clauses. The "even though" clause does not take a comma because it occurs after the first independent clause. Even though indicates contrast.
Analysis: This is also a good revision because the reader can clearly see the relationship between the two clauses.

Run-Ons

A run-on is more than one complete sentence connected without punctuation. Similar to comma splices, writers make run-on errors when they feel that the two complete ideas are closely related. However, complete sentences must be separated with some form of punctuation as the below examples demonstrate.

Let’s revise the following sentence containing a run-on: The ranking system gives colleges a bad reputation it also doesn't allow them to move up in the ranking because of the categories in place.

Option 1:

Separate the independent clauses with a semicolon

Revision: The ranking system gives colleges a bad reputation; it also doesn't allow them to move up in the ranking because of the categories in place.
Explanation: A semicolon signals to the reader that the two independent clauses are closely related in some way but not specifically how.
Analysis: Inserting a semicolon is a good revision because the connection between the two independent clauses is clear to the reader, thanks to the adverb “also,” indicating additional added information.

Option 2:

Separate the independent clauses with a period

Revision: The ranking system gives colleges a bad reputation. It also doesn't allow them to move up in the ranking because of the categories in place.
Explanation: A period signals to the reader that the ideas in the two independent clauses are not strongly related enough to be in the same sentence.
Analysis: Using a period is a good revision too because “also” makes the connection between the two independent clauses clear to the reader.

Option 3:

Separate the independent clauses with a comma plus a coordinating conjunction (FANBOY)

Revision 1: The ranking system gives colleges a bad reputation, and it also doesn't allow them to move up in the ranking because of the categories in place.
Revision 2: The ranking system gives colleges a bad reputation and doesn't allow them to move up in the ranking because of the categories in place.
Explanation: A coordinating conjunction signals to the reader that there is a particular relationship between the two clauses or phrases. And indicates adding more information.
Analysis: Revision 1 is not a good revision because the second “it” and “also” are redundant. Revision 2 is better because it omits the second “it” and “also,” making the sentence more concise.

Option 4:

Separate the independent clauses with an adverbial conjunction plus punctuation

Revision: The ranking system gives colleges a bad reputation; additionally, it also doesn't allow them to move up in the ranking because of the categories in place.
Explanation: An adverbial conjunction signals to the reader that there is a particular relationship between the two independent clauses. A semicolon precedes additionally, and a comma follows it. Additionally indicates adding more information.
Analysis: Similar to the use of a coordinating conjunction in revision 1 above, using an adverbial conjunction in this sentence is not a good revision because it creates redundancy with the repeated “it” and “also,” the latter of which has the same meaning as "additionally."

Option 5:

Separate the independent clauses with a subordinating conjunction plus punctuation if necessary

Revision: While the ranking system gives colleges a bad reputation, it also doesn’t allow them to move up in the ranking because of the categories in place.
Explanation: A subordinating conjunction signals to the reader that there is a particular relationship between the two independent clauses. The while clause takes a comma because it occurs before the independent clause. While indicates two things occurring simultaneously.
Analysis: This is an acceptable revision because the reader can see the relationship between the two independent clauses.

Fragments

Fragments are incomplete sentences that are missing a required subject, verb, or complete idea. Sometimes, they have become disconnected from the main clause (phrase with a subject and verb). Two of the easiest ways to correct them are to insert a missing word or to remove the period between the fragment and the main clause and possibly insert other kinds of punctuation for the newly combined sentence.

Let’s revise the following pair of sentences, the second of which is a fragment that is missing a subject: New ways of transportation have been developed. Making traveling easier and more effective.

Option 1:

Replace the period with appropriate punctuation

Revision: New ways of transportation have been developed, making traveling easier and more effective.
Explanation: When a participle form (a verb in the –ing or –ed form) starts a fragment, attach it to an independent sentence with a comma. Here, “making” is functioning as a reduced verb form.
Analysis: This revision is good because the reader now understands the subject of the second verb “making.”

Option 2:

Insert, delete, or change a required word or phrase

Revision: New ways of transportation make traveling easier and more effective.
Explanation: Because the second clause was missing a subject, you can insert a new one, possibly taking it from context.
Analysis: This revision is also good because not only is the subject of “make” clear, but also unnecessary information has been deleted (“have been developed”).

Option 3:

Make two complete sentences

Revision: New ways of transportation have been developed. This makes traveling easier and more effective.
Explanation: A period plus a subject to complete the fragment signals to the reader that these are not strongly related enough to be in the same sentence.
Analysis: This is a less acceptable revision because the sentences are clear, but they could be more fluid and concise.

Option 4:

Replace the period with an adverbial conjunction plus punctuation

Revision: New ways of transportation have been developed; thus, this makes traveling easier and more effective.
Explanation: An adverbial conjunction signals to the reader that there is a particular relationship between the two complete sentences. A semicolon precedes "thus," and a comma follows it. Thus indicates a conclusion to or result of the previous idea.
Analysis: This is a less acceptable revision because the sentence is clear, but it could be more fluid and concise.

Option 5:

Replace the period with a subordinating conjunction plus punctuation if necessary

Revision: Because new ways of transportation have been developed, traveling is easier and more effective.
Explanation: A subordinating conjunction signals to the reader that there is a particular relationship between the two complete sentences. The "because" clause takes a comma because it occurs before the subject. Because indicates a relationship of reason or cause.
Analysis: This is a good revision because the reader can see the relationship between the two independent clauses.