It is crucial to learn to differentiate between levels of usage; essays and reports require a more formally constructed language than everyday speech. It is all too easy to pepper written work with colloquial phrases or weak constructions that come naturally to mind but are incorrect or inadequate in the context of formal writing.
Language is sometimes rather arbitrarily divided into three major styles: formal, informal, and popular (vulgar). While there is some overlap between them, they operate under different conditions and achieve different ends.
Formal English is, for the most part, a written language. In general, it is confined to the realm of the serious: textbooks, academic or technical works, and most essays you will write at university. You would write formally in a letter of application for a job. Formal language tends to be impersonal and precise, and often uses long, carefully constructed sentences; the formal writer will avoid contractions and abbreviations, and will use a more specialized and complex vocabulary than that employed in everyday speech.
One must, however, beware of excess. Formal writing can easily become incomprehensible because it is too convoluted or wordy. In recent years the proliferation of jargon has become altogether too general. Despite its sophistication, formal language must remain clear.
Informal English is the language spoken by most people every day. While educated speakers retain their knowledge of formal rules, they're more relaxed about grammar and less concerned with vocabulary when they're engaged in ordinary conversation. Informal writing reflects this relaxation. Sentences are shorter, and tend to avoid the more formal punctuation of the semi-colon and colon. Contractions and the first person are acceptable. Newspaper articles and columns are usually written informally, and you use informal language when you write to your friends.
While the dictionary of word usage (16Kb) is designed to help students avoid mistakes in formal writing, it has been prepared in a relatively informal style to make it more enjoyable to read (try taking it on your next holiday). Informal language is not necessarily inferior to formal; it simply serves a different purpose and is directed to a different audience. The essays of Mark Twain,the novels of Margaret Atwood and the film reviews of Pauline Kael are acclaimed works whose style is largely informal.
Popular English can be colourful and highly expressive, but it is out of place in any writing unless you are reporting speech. In everyday speech, imprecision of meaning or poverty of vocabulary can be compensated for by enthusiastic verbal expression, but you can't wave your hands around, or vary your voice for emphasis when you are writing. Many of the errors in usage covered in the dictionary which follows are a result of a lack of awareness of the difference between written and spoken English.
Here are some examples of how vocabulary changes in each level of usage:
A major problem with popular English is that its vocabulary is either too limited or too specialised (not all professors are up to date with the latest slang, and the slang used by one group may be scorned by another). Slang also changes rapidly. The third column in this list will look rather dated in a few years (if it isn't already), while the words in the first two columns have been around since Shakespeare.
The student must always be aware of what kind of work she or he is doing. The style must be appropriate to the subject, the situation and the intended audience. These issues are also important when you decide on your audience or when your professor lowers your grade for using colloquial language.