How can I tell the difference between "an hour" meaning "per hour" versus "in one hour's time"?

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?

• "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

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.

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.

• 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

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.

• 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. Jul 23 '13 at 22:57
• differences not examples Jul 23 '13 at 22:59
• @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