The four ways an English sentence can be built
English sentences are structured in four main ways, told apart by the number and type of clauses they contain. Recognising them shows a writer why some sentences flow better and read more easily than others, and short sentences can be combined into the other types and long ones broken up.
Sentence structure and clarity
The sentence is the basic unit of a written paragraph or text. Correct sentence structure is one way to keep the message of a scientific text clear and concise. If a scientific text is hard to understand, it may be because the sentences do not follow the expected structure and have a confusing number of commas or short phrases. Often, when someone says that a text is heavy or hard to follow, it is because its sentences do not follow the expected structure. A writer who can recognise the four basic types of English sentences will be able to follow some of the main rules about sentence structure and to see why some sentences have a better flow than others and are easier to read. Two teachers of writing courses for PhD students have found guidance on the four types useful there, as it helps the students structure their writing and provides a framework for discussing the text with co-authors. Analysing sentence structure forces the writer to think about the meaning and logic of the sentences, and it takes time but is well worth the effort when the final goal is an informative text that flows well and is easily understood.
Sentences and clauses
A sentence is a set of words that contains a subject, what the sentence is about, and a predicate, what is said about the subject: in Atrial fibrillation is an important factor because its prevalence has increased in western countries, atrial fibrillation is the subject and the rest is the predicate. A sentence includes at least one verb, expresses a complete thought, begins with a capital letter and ends with a full stop, question mark or exclamation mark, and is also called an independent or main clause. A dependent or subordinate clause is a group of words that contains a subject and a verb but does not express a complete thought and is therefore not a sentence, such as because its prevalence has increased in western countries. The four types are the simple, the compound, the complex and the compound-complex sentence, distinguished by the number and type of clauses they contain.
The simple sentence
A simple sentence has only one independent clause, and it contains a subject and a verb and expresses a complete thought, so it makes sense on its own. Its subject can be a noun phrase of several words (most trauma surgeons work full-time), a gerund, which is a verb functioning as a noun (monitoring is essential in diabetes), or a compound subject (the tubes and syringes were washed in saline; cancellation and non-attendance without notice were registered daily), and it often comes at the start of the sentence. Several subjects can share one predicate, as the three subjects do in being overweight, eating unhealthy food, and failing to get adequate exercise all contribute to the risk of developing diabetes.
The verb is the action word and the most important part of the predicate: it says something about the subject and gives information on the aspect of time, past, present or future. It can be a single word (many people worry about global warming) or a verb phrase (samples were collected later). In Both the laser scanner and the modified gauging system performed well for three-dimensional measurements, two noun phrases make a double subject. A predicate can include one verb and a phrase with several other elements.
The data were collected and analysed can be classified in two ways: as a simple sentence with one subject and a compound predicate, two verbs that share the same helping verb and so are both linked to the same subject, or as a compound sentence in which two simple sentences, the data were collected and the data were analysed, are fused to avoid repetition. Whichever way it is classified, what matters is identifying the subject and its verbs, to ensure correct subject-verb agreement. Identifying the subject helps the writer conjugate the verb correctly, as in the finding is or the findings are. The simple sentence is useful for conveying messages, but too many of them one after the other can make the text choppy and boring.
The compound sentence
A compound sentence has two independent clauses joined by one of the coordinating conjunctions known by the mnemonic FANBOYS, for, and, nor, but, or, yet, so, which signal the start of an independent clause. Examples are More children survive to the age of five, and significant progress has been made against infectious diseases and Laser scanners are particularly useful for making three-dimensional measurements, but few hospitals can afford them. The independent clauses in a compound sentence should communicate related messages. Both clauses of the first example are about child mortality, and both clauses of the second refer to laser scanners. In The new procedure was cheaper and more effective, so the committee quickly agreed to implement it in all the outpatient clinics, the first clause explains why the committee agreed. In some cases more than one conjunction can correctly join a pair of sentences, as in The patient was offered a higher dosage of morphine, but (or yet) he chose to remain on the lower dosage.
The complex sentence
A complex sentence has one independent clause and a dependent clause, which depends on the independent clause to have a meaning and adds more information to the sentence. The dependent clause starts with a subordinating conjunction, such as because, until, when or after, or, as a relative clause, with a relative pronoun: who, which, whose, whom or that. In Many people are concerned about global warming because it affects the environment, the dependent clause is because it affects the environment, as it cannot stand alone. In All the university students who passed their exams received a diploma, the relative clause who passed their exams adds the information that not everyone received a diploma, only those who passed the exams. A dependent clause that comes at the start of the sentence usually has a comma after it, especially if it is relatively long, as in Although progress in immunisation coverage has been made, various challenges remain.
In Parents tend to worry and often give their children over-the-counter medications to alleviate discomfort even though fever is a common childhood symptom, the dependent clause begins with even though. In Response rates for individual questions varied because some respondents did not answer every question, the dependent clause begins with because.
The compound-complex sentence
A compound-complex sentence has at least two independent clauses, linked to form a compound sentence, and one or more dependent clauses. The instructions in the two questionnaires differ, and although the meaning is the same, it is possible that patients' responses will be altered has two independent clauses, the instructions in the two questionnaires differ and it is possible, and two dependent clauses, although the meaning is the same and that patients' responses will be altered. The medication should be taken in the morning, but the patients took it in the evening because it made them sleepy has two independent clauses linked by but and a dependent clause starting with the subordinating conjunction because. Compound-complex sentences can get quite long, so too many independent and dependent clauses in the same sentence are best avoided.
Combining short sentences
Three sentences such as Each patient underwent blood tests and CT scans. CT scans were undertaken every three months. The CT scans were reviewed by the same radiologist. are grammatically correct, yet the text does not flow well, because the sentences are disjointed with repetitive phrases. The text would flow better, and the message would be clearer, if the second and third were combined: The CT scans were undertaken every three months and were reviewed by the same radiologist. Combining sentences to make a more compact message sounds simple enough but in practice is not. It requires thinking about the meaning of the messages and how they fit together logically. Altering the linking word or the order of the phrases can make subtle changes to the meaning of a sentence. A different linking word can also change the message in ways that are not so subtle. CT scans were undertaken at varying intervals and were reviewed by the same radiologist gives factual information, while CT scans were undertaken at varying intervals because they were reviewed by the same radiologist suggests that the radiologist may have been available only at certain times.
With practice the four sentence types become easier to recognise, and combining simple sentences into the other types can make a text more interesting and easier to understand. Varying the sentence types can also make a text more interesting. A paragraph of short sentences about osteoporosis can be revised in different ways, and one revision combines People with osteoporosis often have no symptoms. People with osteoporosis have an increased risk of fractures. This is because the bone is weaker. into People with osteoporosis often have no symptoms, but they have an increased risk of fractures because the bone is weaker.
Breaking up long sentences
Long-winded sentences can be broken up into shorter ones to make the message clearer. In worked examples, each long sentence becomes two, mostly by replacing a linking word such as and or so with a full stop and giving the second sentence an opening of its own, as when …can be so difficult and time-consuming and gives some advice on how scientists can cope… becomes …can be so difficult and time-consuming. It gives some advice on how scientists can cope… Once a writer is satisfied with a text, reading it aloud is surprisingly effective for finding problems in it, because the writer will soon hear, or run out of breath, if the sentences are too long or complicated.