Importance: ★★★
Discussion
Presenting a clearly and consistently designed document demonstrates reader awareness. Your paragraphs' appearance is important to a reader who looks at your text, so be consistent in how you signal the beginning of a new paragraph. Make certain that you are familiar with the settings available in your software. If you are using Microsoft Word, for example, have a look at the Paragraph group on the Home tab and investigate your options.
Preferably, a new paragraph in your essay is signalled by an indented line. That means that the first line in your new paragraph is not aligned with the left margin, but rather looks like someone has taken a bite out of it where the new paragraph begins. Alternatively, you may choose to leave extra line space between paragraphs. However, these two approaches should never be combined.
Each of these approaches has advantages. The extra line space produces a roomy page with plenty of white space. The indented line does not; however, it achieves a clear separation between paragraphs, even when a new paragraph coincides with a new page, or after a block quotation. Whichever of these two approaches you choose, you need to stay consistent: pick one and stick to it.
Note that a paragraph is not required to have a certain length to qualify as a paragraph. However, it is important to keep in mind that a paragraph is meant to move your text and your main argument forward. A text that has very short paragraphs (paragraphs of just one or a couple of sentences) will not help you develop your argument; they are more likely to make your text seem fragmented and disorganized. On the other hand, very long paragraphs (say, 300 words or more) may be confusing because of their sheer density.
A rule of thumb is that a paragraph presents one idea, and that presentation typically follows a certain pattern. The paragraph begins with a topic sentence: a sentence that tells the reader what the paragraph is about. Very often, the topic sentence also includes an explicit or implicit link to the previous paragraph. Usually, the topic sentence is a straightforward statement of the idea that the paragraph deals with. The paragraph then goes on to develop that idea by way of examples and explanations. The end of the paragraph is usually the completion of the idea and/or a transition into the following paragraph.
Clearly, the length of a paragraph such as the one described above depends on the complexity of the idea and how much detail is needed in the presentation. For example, a paragraph in an argumentative text may consist of one particular argument, or a counter-argument and its refuatation. In a well-developed paragraph, the idea is fully explained and each sentence is relevant to the topic sentence of the paragraph—and by extension, to the thesis statement of the essay.
Summary
Paragraphs display visually the organization of ideas in your text.
Further Discussion
Paragraphs that contain only one sentence have a rhetorical function: they place a very strong emphasis on the statement made in that sentence. Readers know that a sentence that is given its own paragraph says something very important. Unfortunately, some sources, such as tabloid newspapers and websites, overuse the one-sentence paragraph, partly to produce a sense of urgency and partly to make the text, which is squashed into columns, easier to read. However, when one-sentence paragraphs are used too often, they lose their rhetorical function: if every single sentence is emphasised, no sentence carries extra weight.
Here is what OWL has to say about paragraphs: https://owl.english.purdue.edu/owl/resource/606/01/
For very detailed information on paragraph formatting in Word, see: http://www.mmu.ac.uk/itservices/helpguides/Word-14-FormatText-Paragraph.pdf
Related topics
Linking Expressions, Developing a Thesis Statement