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?
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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:
(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.
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.
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)