Learn English – Difference between “unto” and “to”

[Edited, with examples:] There is some overlap between these terms. People will often even disagree whether a certain expression is best considered informal, or rather colloquial, etc. This is merely an attempt at cataloguing possible associations. If you have suggestions for improvement or refinement, do not hesitate.

Informal:

• This is the broadest, most neutral word. It just means that speech or writing is on the lower side of the formal–middle–informal spectrum. In informal situations, when your conduct is relaxed in all respects and etiquette matters less, you will use informal language accordingly.
• Varieties of language at lower or higher levels in the spectrum are often referred to as lower or higher registers, although the word register is not necessarily about high or low: it can also just be about varieties that are somehow different from the standard.
• Apart from that, it is neither negative nor positive; that's why it is the best term if you don't want to sound disapproving (and if colloquial is not an option).
• There are various degrees of (in)formality: it is usually not a yes–no distinction.
• Although perhaps higher-class people are sometimes expected to be in formal situations more often, and lower-class people are expected to care less about formality, there is no strict relation to perceived class. There are informal words or phrases that are frequently used by perceived 'higher classes' (loo), and, conversely, formal words that are not frequently used by same (Milady, toilet).

Colloquial:

Oxford English Dictionary: 2. spec. Of words, phrases, etc.: Belonging to common speech; characteristic of or proper to ordinary conversation, as distinguished from formal or elevated language. (The usual sense.)

• This is quite close to informal.
• It is mostly used with speech rather than writing, though not necessarily so.
• The word is also slightly stronger on average than informal (i.e. more informal).
• It suggests a yes–no qualification: saying more colloquial is not so common.
• It is usually neither positive nor negative, nor felt to be lower class.
• However, the euphemism "colloquial at best" is often used to mean that it is bad style, referring to a colloquialism used in the wrong setting.
• Because colloquial (and informal) language often varies locally or regionally, while formal language does so much less, it is sometimes associated with provincialisms or regionalisms. But I would consider this contingent, not essential to the term itself.

Slang:

• Slang can be a noun or an adjective; slangy means "resembling or constituting slang".
• It is more often negative than positive—but it can still easily be positive.
• In the formal–middle–informal spectrum, it is more informal than colloquial or informal.
• The word slang itself is a bit informal, while the other words on this page are not.

Oxford English Dictionary: 1. a. The special vocabulary used by any set of persons of a low or disreputable character; language of a low and vulgar type. [notice vulgar used ambiguously]

1. Originally, slang was language associated with low socio-economic class or character, and it is still used with that connotation, though by no means always.
2. A secondary sense has developed, that of general "group talk" in a mildly disapproving or mocking way—even if this group isn't lower class. This sense is now arguably more common than the first. It is often used ironically, as in lawyer slang.
3. A tertiary, entirely neutral sense, "any kind of non-standard group talk", is now commonly used in academia.

Vulgar:

• This means literally "of the people".The Oxford English Dictionary describes its development through the ages:

I. 3. Commonly or customarily used by the people of a country; ordinary, vernacular. In common use c 1525–1650; now arch.

II. 9. Belonging to the ordinary or common class in the community; not distinguished or marked off from this in any way; plebeian

II. 13. Having a common and offensively mean character; coarsely commonplace; lacking in refinement or good taste; uncultured, ill-bred.

• It can now be used to describe language in two ways:

1. The old-fashioned sense is as (II. 9.) above. It is still in use in dictionaries, but less frequent elsewhere.
2. The modern sense is close to (II. 13.), "obscene" or "filthy" to a greater or lesser degree; the lower classes were supposed to be liable to such language, and this sub-sense of (1.) came to dominate the word. So this is obviously even less formal than slang in its lower-class sense. In dictionaries, vulgar could be (1.) or (2.).

Other relevant words:

Jargon: technical or academic language or terminology. This is usually perceived to be somewhat formal, and inaccessible to those outside the field in question.

Vernacular: refers to the native language of normal people, when a different language is used by some groups in society. So this is about different languages, not merely different registers or levels of formality. Usually, the non-vernacular language is used by the educated or higher classes, such as Latin until ca. the 19th century.

I will give a few examples, best description first:

That ain't right.

• slang
• informal
• vulgar (1. of the common people), old-fashioned label
• perhaps colloquial

I will try and convince her.

• slightly informal
• colloquial
• some might call this slang or vulgar (1.), but it isn't felt to be connected with lower class by most people, nor with certain specific groups

That sucks.

• slang
• vulgar (2. obscene), old-fashioned label, because suck has lost its sexual connotation for many people
• vulgar (1. of the common people)
• informal (a bit too general)
• colloquial is possible, but not the best choice

If I was rich, I'd go to London.

• informal
• colloquial, but many people "would never say was", in neither speech nor writing
• slang or vulgar (1. of the common people): probably not

Inspired by a provocative (in a good way) comment by FumbleFingers (above), I did some research into the evolution of the definitions of combustible, flammable, and inflammable in Webster's dictionaries over the past 200 years. The entries for flammable are a bit spotty—absent from the 1806 dictionary, present in the next four editions (1828 through 1890), and then absent from the Collegiate series dictionaries until the Sixth Collegiate (1949). (I expect that flammable appears in the big New International (1909) and Second New International (1934) dictionaries, but those overlap with the Collegiate series, and I find the pared-down listings in the Collegiates a better guide to which words were in widespread, approved use, in the judgment of the Merriam-Webster authorities of the time.) The other two terms are listed in all of the dictionaries I consulted.

Here is a quick summary of how the wording of the relevant definitions has changed over the years:

A Compendious Dictionary of the English Language (1806):

Combustible, a. that will easily take fire or burn

Inflammable, a. easily set on fire, fiery, hot

An American Dictionary of the English Language (1828):

Combustible, a. That will take fire and burn; capable of catching fire; thus, wood and coal are combustible bodies.

Flammable, a. Capable of being enkindled into flame.

Inflammable, a. That may be set on fire; easily enkindled; susceptible of combustion; as inflammable oils or spirits.

An American Dictionary of the English Language (1847):

[no changes]

An American Dictionary of the English Language (1864):

Combustible, a, 1. Capable of taking fire and burning; inflammable.

Flammable, a, Capable of being enkindled into flame; inflammable. [Obs.]

Inflammable, a, Capable of being set on fire; easily enkindled; susceptible of combustion; as inflammable oils or spirits.

Webster's International Dictionary 1890):

Combustible, a, 1. Capable of taking fire and burning; apt to catch fire; inflammable.

Flammable, a, Inflammable. [Obs.]

Inflammable, a, 1. Capable of being easily set on fire; easily enkindled; combustible; as inflammable oils or spirits.

Webster's Collegiate Dictionary, First Edition (1898):

Combustible, a, 1. Capable of taking fire and burning; inflammable.

Inflammable, a, 1. Capable of being easily set on fire; combustible.

Webster's Collegiate Dictionary, Second Edition (1910):

[no changes]

Webster's Collegiate Dictionary, Third Edition (1916):

combustible, a, 1. Capable of combustion; inflammable.

inflammable [same as previous]

Webster's Collegiate Dictionary, Fourth Edition (1931):

[no changes]

Webster's Collegiate Dictionary, Fifth Edition (1936):

combustible, adj. Capable of combustion; inflammable; also, easily excited; irascible.

inflammable [same as previous]

Webster's New Collegiate Dictionary, Sixth Edition (1949):

combustible [same as previous]

flammable, adj. Capable of being easily ignited; inflammable;—preferred by many technical writers and publications to the older equivalent inflammable because of possible misinterpretation of the prefix in- as negative.

inflammable [same as previous]

Webster's New Collegiate Dictionary, Seventh Edition (1963):

combustible adj. 1 : Capable of combustion

flammable adj. capable of being easily ignited and of burning with extreme rapidity

inflammable adj. 1 : flammable

Webster's New Collegiate Dictionary, Eighth Edition (1973):

[no changes]

Webster's New Collegiate Dictionary, Ninth Edition (1983):

combustible [same as previous]

flammable adj. capable of being easily ignited and of burning quickly

inflammable [same as previous]

Merriam-Webster's Collegiate Dictionary, Tenth Edition (1993):

[no changes]

Merriam-Webster's Collegiate Dictionary, Eleventh Edition (2003):

[no changes]

...

I notice three especially interesting things about these evolving definitions. First, two very early definitions (those from 1828) support FumbleFingers' emphasis on the relatively easy and rapid burning behavior envisaged by inflammable: Whereas combustible is associated with wood and coal, inflammable is associated with oils and spirits. This important difference emerges despite previous definitions (in the 1806 dictionary) that offered no indication that such distinctions existed. Just as important, the "oils or spirits" example remains associated with inflammable through the most of the next century (all the way through the 1890 Webster's International dictionary).

Second, though the adverb easily persists in the definitions of inflammable of the middle period (from 1890 until 1963), the explicit inclusion of "combustible" as a definition of inflammable and of "inflammable" as a definition of combustible suggests something approaching interchangeability of the terms during this period, at least in general use.

Third, reemphasis of differences between the two terms begins in the Seventh Collegiate (1963), which clears the way for the new era of (relatively) stark contrast by removing "inflammable" as a definition of combustible for the first time since the 1847 dictionary, and by removing "combustible" as a definition of inflammable for the first time since the 1864 dictionary. The vivid addition to the definition of flammable in the Seventh Collegiate ("capable of being easily ignited and of burning with extreme rapidity") makes the revised definition so much more precise than the Sixth Collegiate's definitions of flammable ("Capable of being easily ignited; inflammable") and of inflammable ("Capable of being easily set on fire; combustible") that I can't help wondering whether the altered definitions of 1963 reflect a sudden dramatic shift between 1949 and 1963 in the way most people used those words, or whether Merriam-Webster's lexicographers were trying to establish a definition of flammable that was consistent with the terminology used by fire prevention authorities, regardless of how contemporaneous nonexpert writers understood the terms combustible, flammable, and inflammable.