Ordering a sentence for reading
A sentence can use every right word and still cost the reader a second pass, because it carries more than the reader can take in at one reading or puts its parts where the reader does not expect them. The reader expects context at the front of a sentence, the stressed material at the end and the subject beside its verb, and reads differently according to a sentence's length, its opening and where its main idea falls.
Why a sentence is not understood
The reason a reader does not understand a sentence might be that it is effectively nonsense, perhaps through a forgotten verb or a misplaced pronoun; that it is too long or convoluted; that particular words in it cannot be understood because they are too long, too specialised (jargon) or not explained (abbreviations); or that it is ambiguous. Ambiguity may come from a grammatical error and also from inappropriate punctuation. Any time a reader has to spend re-reading or puzzling over a sentence causes annoyance at best, and may lead them to give up completely.
How much a sentence carries
A sentence becomes hard to read when it packs in more information than a reader's short-term memory can take in at one reading, and harder still when that information comes in a complex pattern of many subclauses and parentheses. Such a sentence can break no rule of grammar and use only familiar words and still need a second reading. The two factors that make a sentence overwhelming, its length and its structural complexity, both stem from the information the writer tries to present. A complicated sentence reflects a complicated thought; it does not create it. A text that is a struggle to read is frequently difficult because the writer tries to deliver an awkward pattern of thought, not because it contains unfamiliar vocabulary or grammatical errors. Making such a text easier to interpret means rearranging the ideas into more manageable patterns, not just adjusting the wording. Complicated sentences are sometimes necessary for complicated arguments, but all too often a reader struggles with one only because the writer has not considered how easy or difficult the text will be to process.
A 65-word sentence on genetic testing before assisted reproduction would have been easier to process broken into smaller units, arranged to give the main assertion first, followed by the details of the tests and why they are important. A 64-word sentence on antiretroviral therapy would have been easier to process had the writer paused after its bracketed aside, ending the sentence there, to give the reader time to digest the first part of the statement before processing the rest, and that seemingly minor adjustment makes a great deal of difference to ease of processing. Short sentences help make writing clearer because the brain processes information more easily one chunk at a time. The best sentences generally contain one thought, and a sentence that is too long may become barely intelligible. The basic principle is to stick to one main idea in a sentence, plus perhaps one other related point. Long sentences are often full of conjunctions such as and, but, if and however, each of which is like a turn or a fork in a path, and with too many the reader gets lost. A long sentence makes a sequence of events or the logic of an argument harder to follow.
A good average length is 15 to 20 words, which does not mean that every sentence should be 17.5 words long. A sentence of more than about 25 or 30 words, or with more than two commas, should be considered for breaking into two, which usually reads better. A sentence that is too long can still pass a grammar checker: a 63-word sentence did not trigger the one in Microsoft Word. Verbs are the strong words of a sentence, and more than 20 words per verb tends to make a sentence weak, so a sentence that seems weak is checked by counting its words per verb. In one physician-writer's view, a published abstract's results sentence giving the mean scores, between-group differences, confidence intervals and P values for three outcomes is too long, convoluted and stuffed with data, and could have been divided into three or four shorter sentences. A 45-word sentence from a tablet packet, listing the many conditions the tablets relieve, is easier to digest rewritten as two sentences, one of them in the form of a bulleted list.
The topic position and the stress position
The front of a sentence, its topic position, is where a reader expects context and old information, so the context goes before the new information it frames. Research shows that stating the context first lets readers choose a place in their existing knowledge to put the new information that follows, which makes it easier to recall. "Eating broccoli, spinach, collard greens, lettuce greens, rhubarb, Brussels sprouts, kale, beet greens, and carrots can reduce your risk of certain kinds of cancers" gives the information before the context; "The foods you eat affect your health. You can reduce your risk of certain cancers by eating broccoli, spinach…" gives the context first and is more effective. Presenting the context first helps most where the reader, at any skill level, is unfamiliar with the subject, and descriptive headings and subheadings can also give clues to the context.
A reader expects a sentence or paragraph to be about whatever appears first in it, so a passage on the mechanism of action of antidepressants does not begin by talking about beta blockers, unless a really strong connection between the two is made with sensible transitions and related topic strings. The topic positions of a run of related sentences should be consistent, to guide the reader through the paragraph or section.
The end of a sentence, its stress position, is what a reader naturally remembers and emphasises, so the most important or difficult material is often best placed as close to the period as possible and the less important material shifted toward the beginning. Opening with the exciting material and closing on a so-so ending can lose the momentum. When editing, the endings of sentences are worth checking to see whether they can be trimmed for more punch, as our genes largely determine our social behaviour in the way we act in situations we find around us every day trims to our genes largely determine our social behaviour. A technical term, or a familiar but important one, is designed into the end of the sentence that introduces it, where the reader is more likely to take note of it. A sentence that opens The neurotrophins, including nerve growth factor (NGF), … and lists every member before its verb can be revised first to move the list after the verb and then to end on a second sentence, These polypeptides are called neurotrophins, which puts the introduced term at the stress point of the paragraph. Its phrase discrete, and sometimes overlapping is, in the editor's view, a separate thought that should be presented in its own sentence, and its meaning and importance would need to be queried with the author. Placing information where the reader expects it and putting what the reader should remember in the stress position of sentences and paragraphs lets more people read and remember what is written.
The subject and its verb
The subject of a sentence goes as close as possible to its verb, where the reader expects to find them. In English the typical order a reader expects is subject–verb–object, so those parts are kept as close together as possible, and neither the subject and verb nor the verb and object are interrupted with long phrases or unnecessary information. The subject and the verb need to be obvious, because searching for them tires and confuses the reader, and readers tend to skim any text between the two looking for the verb. One limit is to try not to have more than half a line between the subject of a sentence and its verb. Separating them is a common problem for non-native speakers of English, and it can be one for native speakers too.
In the sentence a critical gene that serves as a beacon and gives cells a much needed sense of direction in the chaotic days of early development has been identified by HHMI researchers, by the end the reader has forgotten what was identified. The reader also cannot tell what the subject is, or whether the verb is gives or has been identified. Recast as HHMI researchers have identified a critical gene that serves as a beacon …, the sentence has its verb just after its subject and is easy to understand. The version HHMI researchers have identified a critical gene that serves as a beacon, giving cells a much needed sense of direction during early development also removes the passive.
A sentence in which asthma symptoms is separated by a number of words from improve is weaker than one that puts asthma symptoms improve together, which is easier to understand. Most readers of English can read a passage in which the letters of each word are mixed up, because the first and last letter of each word are in the right place, and the more so if they have some idea of what information is there. A phrase whose words are in the wrong order, by contrast, does not make sense: the reader's eye catches on it and stops, and the reader starts wondering whether a word was meant to be an and, an if, a but or an or and stops thinking about the writer's message.
Mixing short and long sentences
Sentence length is varied to avoid monotony, mixing short sentences with longer ones, because excessive length tires the reader and too many short sentences are also poor writing, boring and choppy. In one reviewer's view, rules on how to structure a sentence and where to put information in it are not as black and white as one plain-English guide suggests, and with long and short sentences, as with the active and the passive, a mixture can work well.
Where possible, readers are best offered a variety of short and long sentences, with the writer mindful of when to choose one or the other. Short sentences give the reader a break from a series of long ones and draw attention to a key idea; as the journalist Tom Wolfe put it, short sentences convey the gospel truth. A short sentence wrapped in white space does so even more. Short sentences slow a reader, deliver a punch, develop drama and clarify complicated material. Long sentences are sometimes unavoidable, as when one noun takes a multisyllabic, multi-word phrase, and then the writer stays mindful of how many such nouns one sentence gives the reader and how many such sentences follow one another. A long sentence is like a librarian with a book added to their arms for every word, and at a certain point it is best to stop adding books and put a period. Long sentences link ideas as they grow, adding texture and cadence and showing that complexity and clarity can coexist. The problem with a long sentence is not its length but length arrived at without intention, through clutter such as are able to, through too many passive constructions, or through missing punctuation and white space that would otherwise control the pace and lighten the visual load of the paragraph. Following a few long sentences in a row with a short one makes rhythm happen; as the writing coach Gary Provost put it, music happens.
Sentences also have a cadence the reader can sense, which is lost when a sentence is convoluted by too many clauses or burdened by too many unnecessary long words. Reading a sentence aloud tests its cadence where that is in doubt.
Openings, emphasis and direction
Sentences are varied in how they begin and in their type, as well as in their length. The same word is not used repeatedly, especially at the beginning of a sentence, and some sentences are simple and declarative, some a little complex and some even begin with prepositional phrases. Three short sentences of a clinical record entry that all begin alike and are all simple and declarative are each proper, but together read as unskilful, and are better combined into one.
The same finding can be stated in ways that change its emphasis: as a sequence in time, first this and now that; with the emphasis on efficacy, the new generation is better than the old ones; as an explicit comparison, survival is better than with the first; and with a small introductory word, such as remarkably or interestingly, added where the result is really interesting or important. Which one to use is settled by returning to the key message: whether the point is that an old approach has been replaced, or that a long-standing practice has now been bettered on survival or on side effects.
Sentence direction is where in the sentence the main idea falls. A loose sentence puts the main idea at the beginning, or lets it stand alone, and its energy typically cools toward the period. Loose sentences suit momentum and pace, but too many of them read like an engine revved and left to coast over and over again. A periodic sentence holds the main idea back until close to the period and builds energy toward it, which puts the main idea in the stress position.