Sometimes my students get a little too hung-up on rules, when it comes to writing essays. Mind you, some rules are vital—if your writing is ungrammatical, readers will have trouble following what you are saying. Other rules (which are really more like guidelines) relate to structure and flow and they also help readers to understand you. Then there are still other rules, which don’t actually contribute much to the reader’s ability to understand and remember your text. These rules aren’t so important. The trick is knowing the difference.
Mind you, there are individual readers and—cough—the occasional rogue professor who care very deeply about this third category of rules, and if you’re writing for one of those people you might as well suck it up and follow those rules, too. But you should still know the reasoning behind them, and why in other contexts it might be okay to ignore them.
You should never use “I” in an academic essay.
Often, when a teacher tells you to “not use ‘I’” or to not use it so much, you can safely interpret this as “I need to give more substance to my opinions by inserting more reasoning and evidence, and possibly more sources, into my essay.” In other words, what this teacher often really means is that you’re asking the reader to believe something just because you said it was so – your essay is full of phrases like “I think…” and “I believe…”.
In other instances, students themselves or their teachers may fear that using “I” makes an essay ‘sound too subjective’ no matter it is used. The truth is, if you are a human being, authoring anything, that thing you author cannot be truly objective. There is a difference between saying, “John’s a fraud,” and “I think John’s a fraud,” and it is intellectually honest to differentiate for your reader what is your opinion or reasoned conclusion, and what is taken from the sources you’re citing. In these cases, using “I” is advisable.
However, it is true that some writers use phrases like “I think” more often than is required by the content – it becomes a kind of nervous tic. In this case, many of the ‘I’s can be safely eliminated or changed.
And remember that you can always find another way to convey that an idea is yours, to keep the ‘I’s from getting excessive or to please a professor who, for whatever reasons, particularly despises the presence of the word ‘I’ (though if you dare you might suggest they try searching it on Google Scholar, to see just how prevalent it is in scholarly journals from every field, including the hard sciences).
Note: Years ago, when scholars were perhaps not quite so resigned to their subjectivity, it was common to assume a sort of royal ‘we’ even when a paper had only one author. This is now frowned upon as misleading. The age of intellectual property has trumped the age of positivism! Nowadays, when an author uses “we” it generally refers to the writer and readers together, as in, “now we turn to a new subject.” Some people like this construction (it makes it easier to avoid the passive voice and nominalizations), and others dislike it (they find the intrusion of writer and reader into the text a distraction from the subject at hand). It’s largely a matter of taste and context.
You should never use the passive voice in an academic essay.
You should avoid split infinitives.
You should always have exactly three main points of support.
Always put your thesis at the beginning.
The answer to all these imperatives is, “Actually, it depends.” If there is any general rule that always applies, it is that a writer should be aware of her purposes and her audience, and suit her structure, style, and language to the particular purposes and audience of a given piece of writing.
The passive voice exists in English because it can be useful – not just to hide the subject of a verb (as in, “mistakes were made”), but also to shift the subject to the end of a sentence, where it may be more convenient for reasons of emphasis or transition (such as “mistakes were made by the President, who is now facing impeachment”).
The notion of avoiding split infinitives is borrowed from Latin, where splitting infinitives can cause confusion. But English works quite differently, and sometimes, in English, not splitting the infinitive can cause confusion. So whether you should do it or not depends on the context.
Grammar Girl has a great guide to splitting infinitives and avoiding them.
The five-paragraph essay model works very well when you’re writing an essay that logically only has three major points of support and only needs to be five paragraphs long. However, for the vast majority of essays that don’t fall into that category, you will have to explore more complicated models.
Putting the thesis at the beginning of an essay has many strong advantages, and seems to work best in any case where the reader is approaching your essay for enlightenment rather than for entertainment or pleasure (you don’t, after all, want to keep your grader in suspense about whether you have something worthwhile to say!). But of course, there are exceptions, and you should always consider the demands of a particular instance when you make such choices.
Often, academic writers put a sort of provisional thesis at the beginning, which tells the reader what to expect without going into detail. This is sufficient to contextualize the information to follow, and fulfills the purpose of assuring the reader that you do, indeed, have a resolution to the problem you’ve set up (that is, that you’re a competent and responsible writer). Then, a more elaborate and specific thesis is stated at the end, incorporating terms and claims that have been made clear in the body of the essay but which were, perhaps, too new to the reader to use effectively in the first paragraph.
Update: See this nice piece from the Smithsonian on rules that aren’t really rules.