I was informed earlier today that the word ago is actually a postposition and the only one of its kind in English. Is this correct? If so, why do dictionaries not use this classification and prefer to label it an adverb instead?
Some sources do list ago as English's sole postposition, but there are actually quite a few other words that pattern the same way:
and the following words, and others, have senses where they follow this usage, although otherwise they are prepositions and/or adjectives:
It's interesting that these all seem to result in phrases that act as adverbs of temporal or spacial extent.
Prepositions are a class of words. The prefix pre- refers to the fact that in languages such as English prepositions occur before their complements — or, as ESL writers and old-fashioned grammars like to say, before their objects:
Typically prepositions take nouns or noun phrases as complements. In the examples above, the park and a hurricane are the complements of the prepositions in and like.
Modern grammar, however, recognises that prepositions can take many different types of complement, or may take none at all. In the following examples we see prepositions which are taking different types of complement:
The first is taking a clause, the second a noun phrase, the third another prepositional phrase and the last has no complement. Previously, these prepositions had been thought to be different types of word in each case (adverbs, subordinating conjunctions, complex prepositions), but many straightforward tests show this is not so (cf. Huddleston and Pullum, 2002, The Cambridge Grammar of the English Language).
Right, so back to words ago and away. These prepositions do not take nouns as complements or as objects. This is easier to show with the preposition away. Consider the phrase:
It is very tempting to imagine here that forty miles is the complement of away, but it is not! Consider the phrase:
Here the complement of away is the preposition phrase from Edinburgh.
If we think about it carefully, the phrase forty miles is modifying the meaning of away; it is not one of the two things that the preposition is showing a relationship between (the two things are: Edinburgh and the place or event that Edinburgh was far from).
Typically, prepositions usually show a relationship between two entities. These entities may be objects, events, times, people or ideas. The thing we are already talking about is usually already referred to in the sentence. The entity we are relating it to is usually the complement or occurs within the complement phrase.
However, this isn’t always the case. With prepositions such as inside the second entity is not always expressed, we mentally reconstruct it:
With the prepositions away and ago the second entity is the place or time we have previously been thinking about: often here or now. As with inside, these entities are usually not expressed in the sentence. The phrases that modify these prepositions do not function syntactically or semantically as the objects of the prepositions but as their modifiers. So, in fact, away and ago are not actually postpositions, unlike those prepositions which in many other languages come after their complements.
However, on the other hand, there is a strong case that in the phrases the world around and the whole day through, that around and through are indeed occurring after their complements, and so could viably be termed ‘postpositions’.
In answer to the problem of why dictionaries do not usually refer to those prepositions which do not take nouns as prepositions - it is because they have so far ignored all of the advances that have been made in modern English grammar! For a good introduction into prepositions, I recommend Bas Aarts, 2011, Oxford Modern English Grammar, which is clear and easy to understand.
I hope this is helpful!
"ago" reminds me of German "vergangen" (passed by). The German question and answer might be" Wie lange ist das her? -- 20 Jahre sind vergangen.
In English this might be: When was it? -- 20 years have gone by.
Etymonline has old forms agan/agone (departed, passed away).
"agone" was shortened to ago. This explains why ago is placed after the time indication. It is still the structure of an old sentence like: Twenty years have/are agone". The new function word ago has the function of a preposition as in German vor 20 Jahren, but it is placed after the indication of the time span. And now dictionaries have the problem of labelling the word class. Inventing a new term for this special word? Not very useful as FumbleFingers says. Etymonline says it is an adjective. Really?? There are other labels which are of no great value. The best thing is to understand the origin of this word, then the word class is of minor importance.
The best would be a special note: ago has the function of a preposition, but is in post-position because derived from a past participle.