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Getting The Combination Right

Constructing complex sentences in research writing

Scientists who struggle with English sometimes stick to short, simple sentences to avoid making grammatical errors. However, a series of short sentences is difficult to read because ideas and arguments do not flow smoothly. Varying the length of your sentences gives your writing rhythm and helps you present your ideas more cohesively. But many scientists fail to join their sentences together properly. In this blog post, I explain how you can join your short sentences together into longer, more complex ones to communicate your ideas more effectively.

Coordinating conjunctions

Words, phrases, and complete sentences can be joined together by conjunctions. And, for, but, yet, so, nor, and or are the only conjunctions that are strong enough join complete sentences together. These are coordinating conjunctions, and they give equal emphasis to each element they join. This means that the ideas on one side of the conjunction are just as important as the ideas on the other side. For example, the following two sentences:

Fibroblasts can be cultured from human skin biopsies. Patient-derived fibroblasts represent a useful tool for investigating the pathological mechanisms of fibrotic skin diseases

can be joined together by and to form one long sentence:

Fibroblasts can be cultured from human skin biopsies, and patient-derived fibroblasts represent a useful tool for investigating the pathological mechanisms of fibrotic skin diseases.

There is nothing grammatically incorrect with the first example; it is perfectly acceptable to present this information in two sentences. However, the two pieces of information are closely related, and it improves the flow of the text to avoid the unnecessary pause and present the two ideas together ­– particularly if the neighbouring sentences are short.

Common coordination mistakes

Commas are often misused with coordinating conjunctions. A comma is needed before a coordinating conjunction that joins two complete sentences together. This comma is important because it tells the reader that one idea is complete and we are now moving on to another. But no comma is needed if a coordinating conjunction adds additional information that is not a complete sentence. In this case, a comma would interrupt your reader by forcing them to pause unnecessarily:

Fibroblast growth was supported in a medium containing 20% glucose, and foetal calf serum was added to the medium to promote cell survival (Two complete sentences, so a comma is needed before and).

Fibroblast growth and survival were supported in a medium supplemented with 20% glucose and foetal calf serum (And is not joining a complete sentence, so a comma here would interrupt the reader).

Another common mistake, particularly for non-native English speakers, is to use and when another conjunction would better communicate the intended meaning. So which conjunction should you use?

  • Use and to add information:

The OCRL1 gene was amplified by PCR, and novel mutations were identified by Sanger sequencing.

  • Use but to indicate a contrast:

The patient was able to sit without support, but he could not walk.

  • Use so to show a result:

The data in 300 of the retrieved studies were deemed poor quality, so these studies were excluded from the meta-analysis.

  • Use for to express a cause:

We screened both sisters for mutations in the BCRA1 gene, for their mother and grandmother had both died of breast cancer.

Only coordinating conjunctions can join complete sentences together as one. Beware of words like however, moreover, therefore, and furthermore. These words seem suitable for joining sentences together, but they are not. For example:

The tumour was localized to the lung, therefore we recommended curative surgery to the patient (Incorrect).

This is an example of a run-on sentence – therefore has been used to join two complete sentences, but it cannot do this with just a comma. The problem can be fixed by using a coordinating conjunction with a comma:

The tumour was localized to the lung, so we recommended curative surgery to the patient (Correct)


or by separating the two sentences with a semi-colon:

The tumour was localized to the lung; therefore, we recommended curative surgery to the patient (Correct).

Subordinate clauses

Adding extra information is a good way to spice up your sentences and stop them sounding monotonous and dull. Clauses that add useful information but cannot stand alone as a complete sentence are called subordinate clauses. This kind of information can be added with subordinating conjunctions – such as after, before, although, because, if, until, when, and whenever (to name a few) – and these conjunctions can serve different purposes. For example, they can show that one piece of information is more important than another:


Histological analysis confirmed the tumour was malignant although the patient was feeling fine (In this sentence, that the tumour is malignant is more important than how the patient feels)


or they can indicate when something is happening:


The patient’s blood pressure decreased after the drug was administered


or they can explain why something has happened:


This is the recommended surgical technique because it is the most commonly reported method in the literature.

Commas and subordinate conjunctions

When you join a subordinate clause to a main clause, whether or not to use a comma depends on which clause comes first. If the subordinate clause comes first, then a comma should introduce the main clause:

Although the underlying mechanism is unclear, the mutation causes Lowe syndrome.

But if the main clause comes before the subordinate clause, a comma is not needed:

The mutation causes Lowe syndrome although the underlying mechanism is unclear.

Relative pronouns

A subordinate clause can also be joined to a main clause with a relative pronoun (which, that, whichever, who, whom, whoever, whose, whosever, and whomever).

Which and that are the most commonly used relative pronouns and are often mixed up. To decide which one to use, simply ask yourself whether or not the information it will introduce is essential to the meaning of the sentence (i.e., is it a restrictive clause or a non-restrictive clause?). If the information is essential, use that and if it isn’t, use which. For example, consider the following sentences:

The drugs that lower blood pressure were administered to the patients.

The drugs, which lower blood pressure, were administered to the patients.

These two sentences have slightly different meanings. In the first sentence, the reader understands that different drugs are available and that only those that lower blood pressure were administered. In the second sentence, the main message is that the drugs were given to the patients and the fact that they lower blood pressure is a non-essential detail.

Which can be used without a comma to introduce a restrictive clause. However, it is often better to stick with that to avoid ambiguity, particularly in research writing.

Correlative conjunctions

These are pairs of joining words that work together to balance words, phrases, or complete clauses in a sentence. The most common ones are:







Not only/but also


When you use these conjunctions as pairs, you must use them properly. Here are some common mistakes to watch out for:

  • Mistaken correlations. This means pairing a correlation with the wrong partner. In research writing, between/to and from/and are often used incorrectly when describing number ranges. For example:


We recruited patients between 2012 to 2015 (Incorrect)


We recruited patients between 2012 and 2015 (Correct)


The drug was administered in doses from 15 mg and 300 mg (Incorrect)


The drug was administered in doses from 15 mg to 300 mg (Correct).


Another common mistake is not specifying the end of a range. For example:


The drug doses in this trial ranged from 15 mg (Incorrect).


Here, it is important to say where the range goes to, as follows:


The drug doses in this trial ranged from 15 mg to 300 mg (Correct).


  • Correlations should not be separated by a comma:


The trial ran from March 2016, to December 2018 (Incorrect)


The trial ran from March 2016 to December 2018 (Correct).


But correlatives can be separated by a comma pair, as the two commas cancel each other out:


The trial ran from March 2016, when the last patient was recruited, to December 2018 (Correct).


  • Correlatives also need to balance. A word put in front of the first word of the pair holds for the whole correlation. For example:


We added the Rac inhibitor to both cortical neurons and to astrocytes (Incorrect)


We added the Rac inhibitor to both cortical neurons and astrocytes (Correct).


Or you can repeat the word after each half of the correlative. For example:


We added the Rac inhibitor both to cortical neurons and astrocytes (Incorrect)


We added the Rac inhibitor both to cortical neurons and to astrocytes (Correct).


  • Correlatives also need to be parallel. This means that the two conjunctions need to join the same thing. This can be tricky to spot. To help you understand, take a look at the following example:


Either patients were treated with the drug or the placebo, but not both (Incorrect).


This sentence is wrong because either and or join a subject-verb combination (patients were treated with the drug) and a noun (the placebo).


Patients were treated with either the drug or the placebo, but not both (Correct).


This sentence is correct because either and or join two nouns (the drug and the placebo).

If you read the two examples again, you will see that the second one is easier to understand. This is because the sentence is parallel.

Complex but cohesive

Using longer complex sentences together with short simple sentences gives your research writing rhythm and makes it more interesting to read. Following the guidelines outlined in this article will help you to construct complex sentences properly in your next research article.

Fit your sentences together properly

Claire Bacon is a former research scientist with professional qualifications in copy editing and medical editing. She edits manuscripts for non-native English-speaking scientists and works as a copy editor for a medical journal.

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