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Good writing requires an appropriate mix of different types of sentence. Too many short, simple sentences can make the writing boring, while too many long and complex sentences can make the writing dense and difficult to understand. This section considers different kinds of sentence, then looks at the three different types of clause used in complex sentences, namely noun clauses, adjective clauses and adverbial clauses. There is an example passage so you can see examples of each type of complex clause in an authentic text, and, at the end, a checklist so you can check your understanding.

Kinds of sentence

There are three basic types of sentence that can be used in English, namely simple, compound, and complex.

A simple sentence is one which contains a single, independent clause, in other words a clause (a group of words with a subject and a verb) which is a sentence by itself, expressing a complete thought. The following are examples of simple sentences.

• Academic writing is difficult.
• It takes a long time to learn.

A compound sentence has two independent clauses joined by a coordinating conjunction (for, and, nor, but, or, yet, so) or a semi-colon. The following are examples of compound sentences, using yellow to show the independent clauses.

• Academic writing is difficult, so it takes a long time to learn.
• Academic writing is difficult; it takes a long time to learn.

A complex sentence, on the other hand, has one independent clause and one or more dependent clauses. A dependent clause is a clause that is not a complete thought and cannot be a sentence by itself. It can be one of three types, described in more detail below, namely noun clause, adjective clause and adverbial clause. The following are examples of complex sentences, using yellow to show the independent clause, and blue to show the dependent clause.

• Some experts do not agree that global warming is a serious problem. [Noun clause]
• Burning fossil fuels adds to global warming, which is one of the greatest threats to mankind. [Adjective clause]
• He learned academic writing quickly even though it is difficult. [Adverbial clause]

Note that it is possible to combine the second and third types to produce a fourth type of sentence, the compound-complex sentence, which contains two independent clauses and one or more complex clauses.

Noun clauses

The first type of dependent clause used to create complex sentences is the noun clause. A noun clause is a dependent clause that functions as a noun. There are three types: that-clauses, wh-word clauses, and if/whether clauses.

That Clauses

That clauses are introduced by the word that, which is often omitted if the meaning is clear. The following are some examples.

• Scientists have concluded that greenhouse gas emissions are a serious problem.
• Experts have warned governments that reduction in fossil fuel consumption is necessary.
• Greene (2016) points out that class size is a key factor in school effectiveness.
• The lecturer assured us that the topic would not be covered on the test.

There are many verbs in English which are followed by that clauses. The following is a list of some of the most common ones. Many of these are reporting verbs, i.e. verbs that are used to report the words or ideas of others, for example when giving in-text citations. They are categorised below into four types, according to whether or not they require an indirect object, and how the indirect object is introduced (with or without 'to'). One of each type is used in the examples above.

In academic writing, verbs are often written in passive voice with the neutral subject 'it'. See the following examples for some of the verbs above.

• It was stated that...
• It is generally agreed that...
• It has been asserted that...
• It has been shown that...

The tense of the verb in the that clause is usually related to the tense of the main verb (i.e. the verb in the independent clause). If the main verb is used in the present tense, the dependent clause verb can be in whichever tense is appropriate. If the main clause verb is in a past tense, the verb in the dependent clause is usually also in a past tense. The exception to this is certain 'base form' verbs, which are always followed by the base form. These include the following: advise; ask; command; demand; direct; insist; propose; recommend; suggest; and urge. There are also adjectives which are followed by base form verbs, such as: advisable; essential; necessary; important; urgent; and vital. The following are some examples to show verb tense.

• It was concluded that lack of attention caused the accident. [Past + past]
• It is agreed that greater attention to the problem will be needed in future. [Present + future]
• It is necessary that governments reduce reliance on fossil fuels. [Base form, reduce]
• It is recommended that greater precautions be taken in future experiments. [Base form, be]

Wh- Word Clauses

Wh- word clauses are formed from wh- questions and are introduced by wh- words such as who, whoever, what, whatever, where, wherever, when, why, which, how, how long, how much, and how many. Since these clauses are statements (not questions), the word order should be subject + verb, and auxiliary verbs used in questions (e.g. do, does or did) are not used. The following are some examples.

• It is not known where the epicentre of the earthquake was. [NOT where was the epicentre]
• The report explains how alpha waves are detected. [NOT how are alpha waves detected]
• It is not known where the substance originates. [NOT where does the substance originate]

If/Whether Clauses

If/whether clauses are formed from yes/no questions and are introduced by the word whether or if. The phrase 'or not' may be added. This can be added at the end of the sentence for both if and whether, or after the word whether (but not after the word if). Note that whether is more formal than if and is therefore more common in academic writing.

• The results of the experiment determined whether the drug entered human trials.
• The results of the experiment determined whether the drug entered human trials or not.
• The results of the experiment determined whether or not the drug entered human trials.
• It is unclear if the measurements were completely accurate.
• It is unclear if the measurements were completely accurate or not. [NOT ... if or not the measurements...]

Adjective clauses

The second type of dependent clause used to create complex sentences is the adjective clause, also called a relative clause. An adjective clause is a dependent clause that functions as an adjective, in other words it modifies a noun or pronoun. An adjective clause begins with one of the following relative pronouns.

• who, whom [for people]
• which [for non-humans or things]
• whose, that [for humans, non-humans or things]
• when [for time]
• where [for place]

An adjective clause should be placed as closely as possible to the noun it modifies in order to avoid confusion. Study the following two examples, where the noun 'The experiment' is modified by the phrase 'that was carried out'.

• The experiment that was carried out on the mouse was unsuccessful. [Meaning is clear]
• The experiment on the mouse that was carried out was unsuccessful. [Meaning is unclear]

Relative clauses are either restrictive (also called defining) or nonrestrictive (also called non-defining). A restrictive clause is necessary because it identifies the noun for the reader. Commas are not used with restrictive clauses. A nonrestrictive clause, on the other hand, is not necessary to identify the noun. It merely gives the reader additional information. As it can be omitted without loss of meaning, it is separated from the rest of the sentence using commas. See the following examples.

• The material which was used in the experiment was impure. [Restrictive, since the clause 'which was used in the experiment' is needed to identify the material.]
• The material from the experiment, which was impure, was weighed twice. [Nonrestrictive, since the clause 'which was impure' merely gives additional information about the material.]

Adverbial Clauses

The third and final type of dependent clause used to create complex sentences is the adverbial clause. An adverbial clause is a dependent clause which is used to modify the verb of the independent clause to give information on when, where, why, how, how long, or how far. It is also used to show contrast or concession (unexpected result).

An adverbial clause can come either before or after an independent clause. If it comes before, a comma is needed to separate the two clauses. If it comes after, no comma is needed. This is shown in the following two examples (adverbial clauses shown in blue).

• Whenever a tectonic plate moves suddenly, an earthquake can occur.
• An earthquake can occur whenever a tectonic plate moves suddenly.

There are several different kinds of adverbial clause. The following lists the different kinds, along with the words that introduce them.

• Time: when, whenever, while, as soon as, after, since, as, before, until
• Place: where, wherever, everywhere, anywhere
• Reason: because, since, as
• Result: so + adjective/adverb + that, such a(n) + noun + that, so much/little + noun + that, so many/few + noun + that
• Purpose: so that, in order that
• Manner/Distance/Frequency: as, as + adverb + as, as if/as though
• Contrast (direct opposition): while, whereas
• Concession (unexpected result): although, even though, though

The following are some examples of each type. The word which introduces the clause is shown in bold.

• Whenever a tectonic plate moves suddenly, an earthquake can occur. [Time]
• Consumers tend to shop wherever they can get the lowest price. [Place]
• Since individuals lack awareness of the issue, the government needs to raise awareness. [Reason]
• The readings were so inaccurate that they had to be discarded. [Result]
• Precautions were taken so that the results would be more accurate. [Purpose]
• The reactants were placed inside the beaker as quickly as possible. [Manner]
• Ordinary writing employs many simple and compound sentences, whereas academic writing uses more complex sentences. [Contrast (direct opposition)]
• Even though academic writing uses more complex sentences than ordinary writing, it should still be clear and easy to read. [Concession (unexpected result)]

Example passage

Below is an example passage. It is taken from the introduction to an article in the BMJ (formerly British Medical Journal). It is used to give examples of different types of complex language in an authentic academic text. The full article, published on 24 January 2018, is available here: https://doi.org/10.1136/bmj.j5855.

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Checklist

Below is a checklist. Use it to check your understanding.