Common mistakes in sentence structure

Sentence structure determines how the different parts of a sentence are put together, from its punctuation to the ordering of its words. As well as following basic word order rules, there are many other things you have to consider to write correctly and clearly structured sentences.

There are two especially common sentence construction mistakes:

  • Run-on sentences: incorrect punctuation used to join different parts of a sentence
  • Sentence fragments: missing necessary components to form a full grammatically correct sentence

Sentence structure is not just a matter of grammar, but also of style and flow. Strong academic writing uses a variety of sentence lengths and structures. It’s important to avoid overly long sentences that can be confusing for readers, but too many very short sentences can make your text feel choppy and disjointed.

Avoid run-on sentences

An independent clause is a group of words that could stand as a full sentence on its own. There are various ways to join independent clauses, but a run-on sentence occurs when they are joined without proper punctuation.

Run-on sentences are a matter of grammar rather than length – even relatively short sentences can contain this error. There are two common mistakes that result in run-on sentences.

Comma splice

Two independent clauses cannot be joined by a comma alone. This form of sentence is called a comma splice.

  • The project ran over the deadline, data processing was extensive.

There are three ways to fix this error. You can split the clauses into two separate sentences.

  • The project ran over the deadline. Data processing was extensive.

You can replace the comma with a semicolon or (if appropriate) a colon.

  • The project ran over the deadline; data processing was extensive.

Alternatively, you can use a conjunction to create a connection between the clauses.

  • The project ran over the deadline because data processing was extensive.

Comma splices can also appear in longer sentences with multiple clauses. In this context they are especially likely to cause confusion.

  • Jimmy likes to take cream and sugar with his coffee, when he drinks it warm, he also likes it black.

Here it is not clear which part of the sentence should be connected to the clause when he drank it warm. Does he like cream and sugar when he drinks coffee warm, or does he like coffee black when he drinks it warm? A semicolon, period or conjunction clarifies the meaning of the sentence, which changes in meaning depending on where the punctuation is placed.

  • Jimmy likes to take cream and sugar with his coffee; when he drinks it warm, he also likes it black.
  • Jimmy likes to take cream and sugar with his coffee when he drinks it warm. He also likes it black.
  • Jimmy likes to take cream and sugar with his coffee, but when he drinks it warm, he also likes it black.

Missing comma with a coordinating conjunction

There are seven coordinating conjunctions in English: for, and, nor, but, or, yet, so (you can remember them with the acronym FANBOYS). When you use one of these conjunctions to join two independent clauses, you need to use a comma before it.

  • Data was gathered through questionnaires and selected respondents participated in interviews.

The missing comma creates a run-on sentence, and like the comma splice, it can often cause confusion. Because we use these conjunctions so often and for so many purposes, it’s useful to know how they are being used when we encounter them.

The comma helps the reader navigate the sentence by signalling that the next part is a new, related, and complete thought.

  • Data was gathered through questionnaires, and selected respondents participated in interviews.

Avoid sentence fragments

A fragment is a group of words that doesn’t contain all the components of a grammatically correct sentence. For a string of words to be considered a sentence, it has to contain a subject and a predicate.

Note that sentence fragments are often used stylistically in journalism and creative writing, but they are rarely appropriate in academic or other formal writing.

Subjects and predicates

The subject of the sentence tells us about the person or thing that acts, while the predicate tells us about what the subject does or is. Put another way, the subject is the noun part of a sentence, and the predicate is the verb part.

Some sentences have more than one subject-predicate combination, but the subject position always comes first. No matter how many subject-predicate pairs come in a sentence, the ratio is always 1:1 –every subject needs a predicate, and every predicate needs a subject.

Subject-predicate examples

Ducks fly.
Haggard and elderly ducks and geese fly slower, lower, and with more caution.
Haggard and elderly ducks and geese fly slower, lower, and with more caution, perhaps because of rheumatism.
Haggard and elderly ducks and geese fly slower, lower, and with more caution, perhaps because their rheumatism hinders them.
Ducks fly; dogs walk.
Ducks fly faster than geese when dogs run and bark.
The dog catches the ball.
The dog catches the ball, which is covered in slobber.
The dog catches the ball, which we bought.
The ball is caught.
The ball now has the following characteristics: a slipperiness, a smelliness, and a chewiness.
The ball now has the following characteristics: it is slippery, it is smelly, and it is chewy.
The ball now has the following characteristics: it is slippery, smelly, and chewy.

Missing predicate

The simplest form of sentence fragment is a sentence missing a main verb. A noun phrase alone is not a sentence – it needs a predicate to be grammatically correct.

  • After they settled the argument, they became friends. A fortunate turn of events.

The fragment can be revised either by using appropriate punctuation to join it to the preceding sentence, or by rewriting the sentence to include a predicate.

    • After they settled the argument, they became friends: a fortunate turn of events.
    • After they settled the argument, they became friends. It was a fortunate turn of events.

    Dependent clause on its own

    A dependent clause has a subject and a predicate, but it does not express a complete thought. It has to be attached to an independent clause to form a full sentence.

    Dependent clauses are often formed with subordinating conjunctions, which include words such as when, after, since, while, although, if, unless, because, while, and whereas. When one of these words is added to the beginning of an independent clause, it turns into a dependent clause.

    • The coast was clear.
    • When the coast was clear.

    The first sentence is an independent clause that can stand as a full sentence on its own. The subordinating conjunction when transforms it into a dependent clause. On its own, this is a sentence fragment. It needs to be correctly connected to another clause to form a full sentence.

    • They would go to safety. When the coast was clear.
    • They would go to safety; when the coast was clear.
    • They would go to safety when the coast was clear.
    • When the coast was clear, they would go to safety.

    Note that these clauses cannot be joined with a semicolon. A semicolon can only join two independent clauses.

    Misuse of the present participle

    The present participle is the form of a verb that ends with -ing (e.g. running, researching, being). Sometimes it is misused where a present or past simple form should be used instead. An -ing verb on its own can be part of a modifier that refers to another part of the sentence, but it can’t mark the beginning of a predicate.

    The most common verb abused with this mistake is to be, which is conjugated as being when it should be conjugated is or was.

    • He argued all night long. The point being important.

    The point being important is a sentence fragment. It needs to be connected to another clause, or revised with a properly conjugated verb.

    • He argued all night long. The point was important.
    • He argued all night long, the point being important.

    What can proofreading do for your paper?

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    Split up overly long sentences

    Sometimes a long sentence is grammatically correct, but its length makes it difficult to follow. To make your writing clearer and more readable, avoid using too many overly long sentences.

    The average sentence length is around 15-25 words. If your sentence starts to exceed 30-40 words, you might want to consider revising it. Removing redundancies and inflated phrases is a good way to start, but if all the words in the sentence are essential, try to split it up into shorter sentences.

    Because early childhood, defined as the first five years of one’s life, is a critical time in terms of not only physical but also social development, studies have focused a significant amount of attention on this period, and this section discusses researchers’ key findings and their implications for the current study.

    This sentence doesn’t contain any grammatical errors, but the information can be presented more clearly by revising its structure.

    Early childhood, defined as the first five years of one’s life, is a critical time in terms of both physical and social development. Hence, studies have focused significant attention on this period. This section discusses researchers’ key findings and their implications for the current study.

    Another issue to watch out for is overly long introductory phrases or clauses. If your sentence starts by repeating material that has already been presented, it can bury the new information you want to communicate.

    In contrast to what the previous section on the data’s collection, analysis, interpretation, and implications might suggest, none of the findings were significant.

    The main point of the sentence is that none of the findings were significant, but the long introductory clause distracts us from this information. To clarify the point and shorten the sentence, focus on reducing repetition.

    In contrast to what the previous section might suggest, none of the findings were significant.

    Shorter sentences are generally clearer and more readable, but using too many very short sentences can make a text feel choppy, disjointed or repetitive. Try to use a variety of sentence lengths, and use transition words to help readers see how your ideas fit together.

    Baseline data were collected on day one. Data were also collected after one week. The measurements were taken at 9am. This was done every day.

    While all of these are grammatically correct sentences, the text reads more smoothly if they are merged.

    Baseline data were collected on day one and after one week. The measurements were taken at 9am every day.

    Other sentence structure tips

    Apart from these basic rules, there are some other techniques you can use to improve your sentence structure.

    Use parallelism to balance your sentences Fix dangling and misplaced modifiers

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    Shane Bryson

    Shane finished his master's degree in English literature in 2013 and has been working as a writing tutor and editor since 2009. He began proofreading and editing essays with Scribbr in early summer, 2014.

    3 comments

    Ry Capati
    December 18, 2018 at 10:53 PM

    Awesome job!

    Reply

    Raimo Streefkerk
    Raimo Streefkerk (Scribbr-team)
    December 20, 2018 at 9:56 AM

    Thanks Ry!

    Reply

    Ben Burleigh
    January 26, 2018 at 7:51 PM

    Great job! Clear, succinct, real!

    Reply

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