I was just taking this grammar test and there's this question on it:
Which of these sentences are correct:
Aren't they both correct? I make ten phone calls a day probably means I make 10 phone calls per day, and I don't know how to describe what the other sentence means other than just to use I make 10 phone calls in a day. Is there a difference in their meaning?
Changing the content may make this clearer. If I asked you how fast you ran the 100 metre dash, you may say "I ran ten metres a second" but you wouldn't say "I ran ten metres in a second". The former talks about what you did throughout the race while the latter talks about what you did in one second of the race.
Sentence #1 is clearly correct and natural.
I wouldn't say that #2 is grammatically incorrect (I can imagine contexts where it would be OK), but it does sound a bit off. I think the reason is that the habitual aspect of "I make" conflicts with the specificity of "a day".
To see this, let us put both sentences in the past tense, and consider:
What do these mean? #3 talks about a habitual past action: "Back then, I was such a good salesman -- I made ten phone calls a day, and my boss always told people I was his most valuable employee."
In contrast, #4 talks about one past action on one particular day: "I was a pretty good salesman back then, but then this one day I was simply on fire: I made ten phone calls in a day!"
From this we can see why #2 doesn't quite work: "a day" here suggests a perfective aspect (one event which is completed), but "I make" is a habitual present. You could say "I can make 10 phone calls in a day", though.
(By the way, as a native speaker, your analysis feels totally natural to me, including your difficulty in putting your finger on how exactly to describe what #2 means -- I went through precisely the same thoughts.)
Both sentences are grammatically correct but the first one makes much more sense.
I make ten phone calls a day.
This means that every day I make ten calls. It may be an approximation or average. It describes a continued experience, and would be an answer to the question "how much do you use the phone in your job?". It is a meaningful statement for many people.
I make ten phone calls in a day.
This would make sense as an answer to the question "if you have to make ten phone calls, how much time does it take?" It does not describe a continuous experience and does not mean every day. It is probably not a sentence many people would use.
Since this is a test, the likely answer is #1.
