Formal and Informal English

The difference between formal and informal English is not a difference between correct and incorrect, but a difference of what is known as register. A register is a variety of language related to a particular subject matter or area of activity, a set of words and expressions as well as syntactical features that may be said to characterise that specific area of language. There are many registers: technical, academic, mathematical, scientific, etc. Very broadly speaking, we can also speak of a “formal” and “informal” register in English. In writing academic reports and the like, it would be normal to draw most of the vocabulary and expressions from the formal register, and few, if any, from the informal. This entails avoiding colloquial (everyday) or slang expressions in your writing assignments.