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The "an" can be used as a preposition with the meaning of "per": My rate is $10 an hour.

Also it can be a used as a determiner: I will be ready in an hour.

How can I tell these 2 apart?

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  • "An" is used as a determiner in both cases. In the first example "a(n) X" it is used following a quantity without a preposition, in the second it is used with a preposition. Jul 24 '13 at 1:03
  • Are you saying that "per" is missing but implied? Jul 24 '13 at 1:44
  • No. Nothing is "missing". I'm saying that "per" is not required: that's what the construction means. "Per" is a different way of saying the same thing. You can also say "each hour", "every hour". Jul 24 '13 at 1:55
  • It's not really "missing", but you can easily assume the elided preposition "My rate is $10 for an/one/each hour." Jul 24 '13 at 3:30
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It's the same for any word that has multiple meanings; you have to determine it from context. As you said, an can mean something similar to per, or it can be used to mean a/one. If you take your two examples and substitute the 'definitions' instead of the word an in each case, it is quickly clear which makes sense and which doesn't:

My rate is $10 per hour.

Meaning: "I charge $10 for each hour" - makes sense

My rate is $10 one hour.

Meaning: Nothing; this doesn't make sense as an English sentence. So you know this isn't the correct meaning.

I will be ready in one hour.

Meaning: "When one hour has passed, I will be ready" - makes sense

I will be ready in per hour.

Meaning: Again, nothing; this just doesn't make sense.

So as you can see, if you know multiple meanings of a word, you can substitute each meaning into the sentence and see if the sentence makes sense. That way you can tell from the context which definition applies.

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Substitute per for a(n). If it's ungrammatical, it can't mean per.

My rate is $10 an hour.
My rate is $10 per hour.

This example works.

I will be ready in an hour.
*I will be ready in per hour.

This example does not. In this sentence, the preposition in needs to be followed by a noun phrase; an hour works as a noun phrase, but per hour is a preposition phrase, so it's ungrammatical.

In other words, you can distinguish the two syntactically.

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  • Oops, I see that this is more or less what WendiKidd's answer says. I'll go upvote that answer :-)
    – user230
    Jul 24 '13 at 0:17
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It's entirely by context. In the first case you have a unit $10 and a measurement the "hour". In the second you're specifying a time allotment.

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  • Well, how is the context different? Can you name formal differences? Jul 23 '13 at 22:56
  • Formal references? Example 1: I'm driving at 50 miles an hour. Example 2: I'm going to leave here in an hour.
    – Jacobm001
    Jul 23 '13 at 22:57
  • differences not examples Jul 23 '13 at 22:59
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    @bonomo As WendiKidd points out, there is no preposition in [measure1] a(n) [measure2]: [$10] an [hour], [35 miles] a [gallon]. The default parsing of the idiom is that [measure1] is divided by one [measure2]. Jul 24 '13 at 0:58
  • @StoneyB, any other cases you can think of? I need to teach a program how to tell these two apart so I need as many clues as possible. Jul 24 '13 at 1:41

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