Formal or Informal?

3. Long Sentences or Short Sentences

What we said about long and short words applies to sentences too. Academic writing tends to use long sentences, but it's also a good idea to vary long sentences with short ones. A short sentence coming after a few long ones has a lot of impact.

Sometimes you can use a short sentence to give an illustration of what you've just said in a long one:

In many cases, however, despite the existence of general conventions it remains a writer's personal decision whether to use a comma or not. Some people like using commas and others prefer not to. (Fabb, N. and Durant, A. (1993) How to Write Essays, Dissertations and Theses in Literary Studies.)
(A sentence of 23 words, followed by one of 10 words. The second sentence reinforces and clarifies the first)

Sometimes you can use a short sentence to contrast with what you've said in a long one:

Normally you would paraphrase rather than quote, because this is a way of rewriting the relevant material in a way which assimiliates it into your essay (rather then, as with a quotation, leaving it sticking out because of its different style). But you should use a quotation if the language of the original is itself important. (Fabb and Durant, 1993)
(Note how a sentence of 41 words is followed by one of 15 words. The contrast is introduced by the word 'But', and reinforced by the short sentence following the long one.)

So, as both long words and short words have their uses in academic writing, so do long and short sentences. And mixing them together makes for lively writing.