The "an" word is usually a determiner: I will be ready in an hour. It can also be used as a preposition with the meaning of "per": My rate is $10 an hour.

How can I tell in each particular situation what "an" is?

  • Please do not crosspost your questions to several SE sites. I guess, it better fits at ELL, so voting to close this one as offtopic.
    – bytebuster
    Jul 24 '13 at 1:35
  • I am sorry if there is more than one SE that the question fits. Linguistics is where it belongs, but chances are it will be better answered in ELL. Jul 24 '13 at 1:40
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    @bonomo As far as I know there's no rule against cross posting but it tends to be inefficient if you have two groups working independently on the same problem at the same time (also it's a bit of a dick move). I think you should choose one site to ask it on and then if you don't get a satisfying answer you can take the information you gained from the last question, incorporate it into your question, and ask it again. No matter what you should always include a link to other versions of the same question so we can at least see what other people are saying.
    – acattle
    Jul 24 '13 at 3:01
  • In my past experience when it comes up, direct cross posting is not considered great but asking versions of the same question tailored to the field of each of multiple sites is not bad and can result in quite complementary answers. That said, I'm not sure this is really a linguistics question as currently worded. If it's more about usage it belongs on another site. If it's about word classes and polysemy it should be edited a bit and then it belongs here. I definitely agree about linking the two questions though! Jul 24 '13 at 3:17
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    @acattle Just seen this but actually, the problem with crossposting exists and you can read this to see why it's bad. There's nothing wrong with posting a question on multiple sites as long as: (1) You let some time pass between the two questions. (2) You tailor the question according to the site. ... Now, if your questions fits many sites, then the question is too broad and not really fit for any of them. I'll leave this question open however because it's been answered and we can always answer it "linguistically".
    – Alenanno
    Jul 25 '13 at 18:01

The answers so far agree that a/an must be a preposition because it can be replaced by other prepositions. I think this is a possible but not ideal analysis. It is not ideal because it means positing a new preposition a/an which only occurs in phrases of the type [x amount of currency] a/an [(time) unit], where 'unit' is a measure of something that can be bought - hours of working time or things, for example (thanks to dainichi for pointing out Ten bucks a piece). The description of a grammar and vocabulary of a language should be parsimonious, which is to say it should have as few entries in the lexicon/vocabulary and syntactic rules as possible as long as this results in an equally accurate analysis.

I think the phrase [x amount of currency] a/an [(time) unit] might have arisen due to ellipsis from [x amount of currency] for a/an [(time) unit]. This explanation avoids positing a new entry of a/an as preposition.

Also note that such expressions are common in other languages:

  • German: zehn Euro die Stunde (< zehn Euro für die Stunde)
  • Gloss: ten euro the hour
  • German: zehn Euro das Stück
  • Gloss: ten euro the piece
  • French: dix euro l'heure
  • Gloss: ten euro the-hour

(Not 100 % sure about the French example since I'm not a native speaker, but I think it's grammatical).

The only difference is that German and French use a definite article, whereas English has the indefinite article. In neither of these languages, I think, do articles/determiners double as prepositions.

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    "only occurs in phrases of the type [x amount of currency] a/an [time unit]". How about "ten bucks a piece"?
    – dainichi
    Jul 25 '13 at 4:10
  • Or "five-a-side football"? Jul 25 '13 at 6:19
  • Thanks for pointing out these cases. I changed my answer to cover 'ten bucks a piece', which I think can be accounted for by broadening the definition of the phrase a bit. I don't know what 'five-a-side football' means, though? Could you maybe explain, hippietrail?
    – robert
    Jul 25 '13 at 8:58
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    Some even have Wikipedia pages: Five-a-side football, Nine-a-side footy. Jul 26 '13 at 1:41
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    Thanks for the explanation. I would still maintain that these cases can be explained with ellipsis from "five on a side" etc. It is of course possible that a will be regrammaticalised as a separate preposition from these uses, but that seems a long way off. Outside these few frames where it can be used in this way, it is still ungrammatical afaik: * We need two tickets a person, *He gave us five spoons a plate, *I'd like one piece of cake a child.
    – robert
    Jul 26 '13 at 12:04

This is a question of syntax. Usually you find or think up a bunch of diagnostics.

The easiest here is "If I replace an with per does the sentence still make sense?"

  • My rate is $10 per hour.
  • I will be ready in per hour. *

The first sentence is grammatical so "an" was a preposition.
The second sentence is ungrammatical, indicated by the * symbol, so "an" was an article.

  • I don't think he was really asking "when is 'an' an article and when is it a preposition?" I think he's asking "how can 'an' be an article sometimes and a preposition other times."
    – acattle
    Jul 24 '13 at 3:06
  • Hmm it's hard to tell - it's not a very good question. I was just going to comment but the comment got too big so I made it an answer. Jul 24 '13 at 3:13
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    I reread his question and I must have missed the last sentence the first time. In my defense, the title implies my original interpretation while the body implies your interpretation.
    – acattle
    Jul 24 '13 at 3:30

The simple answer is that there are two "a/an"s in English. One is an article and one is a preposition.

When you consider that languages are full of homonyms it is natural that there can be multiple items in a lexicon that share the same form (pronunciation/spelling) while having different meanings. Consider the word "sanction" which can mean the exact opposite of itself depending on whether it is a noun or a verb yet we have no problem understanding it. This is a good comparison because like "a/an" the type of word it is interpreted as depends on how it is used in the sentence but "a/an" has the added advantage that, unlike "sanction", it does not carry a semantic value (or at least none beyond the logical implications of using an indefinite article).

When trying to decide if a particular instance of "a/an" is an article or a preposition you can use a substitution test like the ones put forward by hippietrail: substitute "a/an" in a sentence with other articles or prepositions and see if the sentence is still syntactically valid (it might be semantically weird but that's OK)

  • A/An is a specialization of one, and articles are used in many idiomatic constructions. This is one. $10 an hour is short for $10 one hour, which is also clear, as is one hour $10. One of the NPs has to be an amount of money and the other has to be an amount of time. Either can be an article if the number is 1. A dollar an hour.
    – jlawler
    Jul 24 '13 at 3:29
  • I would say this is idiomatic but $10 one hour doesn't sound so natural to me. Compare a non-price usage such as Five a side football sounds fine but Five one side sounds wrong. Jul 24 '13 at 3:44
  • @jlawler I've never seen "$10 an hour" as idiomatic since, at least for me, "$10 one hour" seems strange unless I render it as "$10, one hour" or "$10 for one hour". The inversion "one hour $10" is even worse, being malformed unless I perform similar alterations. Perhaps I've lexicalized the phenomenon.
    – acattle
    Jul 24 '13 at 3:44

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