We know the differences between formal and informal English from middle school: formal English is used for serious situations like professional correspondence, official documents, news reports, et cetera; informal English for everyday conversation and in relaxed settings. There’s also the unofficial semi-formal English, a melange of formal and informal.
But that was then, and this is now. In 2016, and on the web especially, the two become less rigid, and can be used interchangeably. Rules are more lax, and many writers will use informal language to be more relatable and accessible to readers. So what does this mix of styles look like? And how do you take the best parts of both to achieve the effect you want?
Formal English has an inflexible structure. Sentences tend to be longer, and the vocabulary more “five-dollar”. Informal has shorter, sometimes run-on sentences, and allows for slang and more sloppy structure. For instance, you can use “five-dollar” to describe words in informal writing, whereas in formal writing you cannot. Conversely, words like “whereas” and “conversely” are typical markers of formal writing.
Now, the previous paragraph was written in a style that obviously leans to the informal side, at best it’s semi-formal with formal features. For writing on the web, this style is perfect. Simple enough to be understood by any English speaker, but not so simple as to alienate high-brow readership.
This is what you need to aim for.
One-sentence paragraphs like the previous one are also markers of informal-style writing on the web. It easily jumps into the reader’s eye and both its brevity and isolation from the other paragraph gives it impact.
If you’re writing on a subject that demands that readers trust you, use more formal writing: longer, more complex sentences, as few contractions as possible, and absolutely no informal constructs (i.e., “less” when you mean “fewer”). Avoid unclear references (“this”, “they” when referring to a subject of a preceding sentence), clichés and metaphors, as well as first-person writing when it’s avoidable.
As an English speaker, you already know when to use formal and informal writing. An email to a prospective employer? Definitely formal. An email to a coworker at your level? Informal is acceptable. When deciding whether to lean on the formal or informal side, consider the audience. If it’s regular people who just need to absorb some kind-of-important information, formal style might be too much. If you’re presenting information where precision is key, you’ll need formal writing: to-the-point, complex sentences, each with their own point. Always mind the audience.