A sentence is a group of words that communicate a complete thought. In formal writing, there should be at least one subject and one main verb to form an independent phrase.
A sentence can be short and simple ( 1 independent clause) or long and complex ( at least 1 independent clause and 1 dependent clause).
Long and complex: Ionization detectors respond more quickly to flaming fires with smaller combustion particles while photoelectric detectors respond more quickly to smoldering fires.
A sentence should ‘flow’, i.e. two or more phrases might be combined.
A sentence can add or list information, provide similar or opposing ideas, compare information, describe, define and/or combine ideas.
In this section, a number of grammatical terms will be referred to. If you feel unsure about their meaning or what they refer to exactly, take a look at our sister-site, EngOnline, a web-based grammar tool with information and exercises.
What is a sentence?
A sentence is a group of words that communicate a complete thought. This could be, for example, a statement, a question or a request. A complete sentence must contain at least a subject and a main verb:
|Engineers (subject) often rely (main verb) on models.|
A complete sentence must be an independent phrase. Compare the following:
Both sentences have a subject (engineers / they) and a verb (rely / facilitate). The first sentence is complete; i.e. it can stand alone (independent). The second sentence is different because it contains a subordinating conjunction (since) and so needs more information to complete it/complete the thought. A complete version of the sentence might read:
|Engineers often rely on models, since they facilitate the design process.|
Some other subordinating conjunctions: after, although, as, because, before, even if, even though, if, in order that, once, provided that, so that, though, unless, until, when, where, whether, while
So, a complete sentence needs a main verb, which also means a main clause, i.e. an independent clause and this might involve more than one clause together if you are joining two or more ideas together in one sentence.
Why worry about different kinds of clauses? Well, we tend to only use dependent clauses on their own in informal situations, particularly in dialogue. Here at Chalmers, most of your writing is academic writing which is a formal style of writing. Remember to always take purpose, audience and text genre into consideration when deciding on the level of formality required. With sentences, it is not only a case of using complete sentences, but also how you combine them and how complex you make them, but also making a judgement on the textual context.
Common Problem Areas
Fragments or hanging sentences: these are other terms for incomplete sentences. You may be missing a main verb, a subject, or it’s only a dependent phrase and so the idea is incomplete.
Run-on sentences/comma splices: these are sentences where punctuation, usually a comma, has divided up complete sentences (comma splice) instead of a full-stop being used. Consequently, complete sentences follow each other in a continuous string (run-on), which is not strange in relation to informal, dialogue-type language, but it is not appropriate for formal language.
|Run-on: Aluminium is a metal, it is abundant, it has many uses.
Formal: Aluminium is an abundant metal. It has many uses
There is so much more to writing a good, readable text than making sure all your sentences are complete, i.e. have a subject, verb, and are independent statements. The length of a sentence is also important. Sentences can be short and simple (i.e. an independent clause) or long and complex (a combination of independent and dependent clauses). They often combine more than one phrase and more than one piece of information into a more informative sentence. The examples below in Figure 1 illustrate how several simple sentences can be combined. Consider which option is easiest to understand and most effective in conveying its message.
|Type of sentence||Single sentences||Combined sentences|
|Addition (the sentence adds information to the last sentence)||Smoke detectors can be installed on the ceiling. They can also be mounted along high walls near the ceiling||Smoke detectors can be installed on the ceiling or mounted along high walls near the ceiling.|
|Listing||Ionization detectors have an ionization chamber. They also have a source of ionizing radiation.||Ionization detectors have an ionization chamber and a source of ionizing radiation.|
|Similarity||Ionization detectors are effective smoke detectors. Photoelectric detectors are also effective.||Both ionization and photoelectric detectors are effective smoke sensors.|
|Opposition (the sentence shows how two items are different)||Ionization detectors respond more quickly to flaming fires with smaller combustion particles. Photoelectric detectors respond more quickly to smoldering fires.||
Ionization detectors respond more quickly to flaming fires with smaller combustion particles while photoelectric detectors respond more quickly to smoldering fires.
|Parallelism (the same structure introduces each item in the sentence)||The devices may be powered by a 9-volt battery. They may also be powered by a lithium battery or 120-volt house wiring.||
The devices may be powered by a 9-volt battery, lithium battery, or 120-volt house wiring.
|Description||The second type of smoke detector is a photo-electric detector. It uses a projected beam of light in a special chamber.||The second type of smoke detector is a photo-electric detector which uses a projected beam of light in a special chamber.|
|Definition||Smoke is not just a cloud of gas. It is actually made of very small carbon particles suspended in the air.||Smoke is not just a cloud of gas but consists of very small carbon particles suspended in the air.|
Figure 1. Sentence Combinations (Adapted from Helmenstine, 2003)
These different relationships are exemplified in the following examples, both as separate sentences and linked. It is clear that in some cases the sentence is better combined, in others, better as a single sentence, and sometimes it makes no difference. Keep in mind that the choice of whether or not to combine sentences may depend on the surrounding text.
Many small, single sentences
There are ionization smoke detectors. There are photoelectric smoke detectors. Smoke alarms warn of fires. They can use ionization detectors. They can use photoelectric detectors. Some alarms use both detectors. Sometimes alarms also use a heat detector. The alarm can be powered by a 9-volt battery. The alarm can be powered by a lithium battery. The alarm can be powered by 120-volt house wiring.
One long sentence (not recommended)
|The two main types of smoke detectors, ionization detectors and photoelectric detectors, use one or both methods (sometimes plus a heat detector), to warn of a fire and may be powered by a 9-volt battery, lithium battery, or 120-volt house wiring.|
There are two main types of smoke detectors: ionization detectors and photoelectric detectors. A smoke alarm uses one or both methods, sometimes plus a heat detector, to warn of a fire. The devices may be powered by a 9-volt battery, lithium battery, or 120-volt house wiring.
Figure 2. Sentence Combinations (Adapted from Helmenstine, 2003)
All the versions can be understood by most readers but some are easier to read than others. Example 1 is rather repetitive and clumsy while Example 2 takes some concentration as it is easy to lose the sense of the sentence. Example 3 is more effective in communicating its message. Looking more closely at and comparing Examples 1 and 3, by combining many information units into one sentence, the structure is more effective when avoiding repetition and clumsy expressions.