In any language, different styles of expression are appropriate in different situations. We can go from the formal to the informal, the written to the spoken, from technical language (or jargon) to slang.
There are no "rules" as such; nevertheless, there are plenty of features which distinguish formal styles from informal styles. Here are some of them.
Principles of English written style:
Note: these are principles: they are by no means to be considered as "rules".
a) The more formal a document is, the more it will use inanimate nouns as subjects of a sentence.
b) The more formal language is, the more it is likely to use passive structures.
c) The more formal language is, the more verbal nouns it will use.
d) The more formal a document is, the more words of Latin origin it will use.
Conversely
a) The more informal or spontaneous language is, the more it will use humans as the subjects of sentences.
b) The more informal a text is, the less it will use passive structures,
c) The more informal a text is, the more it will use verb structures (where a choice is possible) instead of verbal nouns.
b) The more informal or spoken a text is, the more words of Germanic origin it will use.
Here are some examples; in each case, the same idea is expressed using three different levels of formality: look at the different changes that occur, as we move from a formal style to an informal one.
Example 1:
Please await instructions before dispatching items.
Please wait for instructions before sending items off.
Don't send anything off until you're told to do so.
Example 2:
Essential measures should be undertaken at the earliest opportunity.
One should undertake any necessary measures at the earliest opportunity.
You should do whatever you have to as soon as you can.
Example 3:
Prior to the discovery of America, potatoes were not consumed in Europe.
Before America was discovered, potatoes were not eaten in Europe.
Before they discovered America, Europeans didn't eat potatoes.
From Written to spoken styles:
Written and spoken versions of a language use different styles, different registers. To talk in "written English" may be no more appropriate than to write using a "spoken" variety of English. Generally speaking, written English is always more formal than spoken English. Nevertheless, there are informal forms of written English (notably in fiction and in the popular press), and formal styles of spoken English, in particular "discourse", or prepared speech.
In the following examples, the same message is expressed in five different styles, from an extremely formal written style, to the very informal spoken style.
Note in particular how the colour coded word groups evolve.
(The information expressed in the following examples is necessarily quite technical, as formal jargon levels of expression are totally inappropriate for non-technical communication).
Example 1:
a) Jargon, very formal. This is the style of language used in official reports, technical studies, etc. It is exclusively a style of written English, full of verbal nouns, technical words and passives.
Consequent to the appreciation in the exchange value of Sterling against other currencies, necessary fiscal measures were introduced by the government in order to reduce the likelihood of an import-led consumer spending surge.
b) Written, formal, clear.
This is clear, written English, as found in the press or in documents aimed at ordinary educated readers.
After the international value of Sterling rose, the government was obliged to take fiscal measures to reduce the likelihood of a surge in consumer spending led by cheaper imports.
c) Written style for the general public, discourse, scripted radio or TV news style.
As the value of Sterling increased compared to other currencies, the government was forced to take tax measures to head off a rapid increase in consumer spending spurred on by cheaper imports.
d) Formal spoken style – radio, seminar, talk.
As Sterling's international value went up, the government had to take tax measures to head off a consumer spending boom spurred on by cheaper imports.
e) Relaxed, informal spoken style: discussion.
There is plenty of use of prepositional verbs. All actions are now expressed through verbs, not verbal nouns
As Sterling went up in value, the government had to put up taxes to stop consumers splashing out on too many cheap imports.
f) Relaxed, simplified, chat, very informal spoken style;
Note the addition of repetition and fillers.
And you see, Sterling went up and up in value, so as a result, the government had to go round putting up taxes, you see, to stop everyone going out and splashing out, spending all their cash on cheap imports.
Example 2:
Citizens whose normal place of abode is out with the United States of America are henceforth required to register their interests with the United States consulate nearest their domicile ; failure to register may lead to forfeiture of fiscal exemptions on revenues paid by sources in the United States.