I know it is better to just say before noon, but if you wanted to say "before the afternoon arrives" or "before the afternoon ends" would it be correct?
There's nothing ungrammatical about them, but these expressions sound rather unnatural to my native ear.
As to a better way to word it, that depends on the context. Am I talking to a coworker, about when we should meet with a client? To the appliance store, about when they are going to deliver my new washing machine? To some friends, about when we are going to meet for drinks?
In the workplace, I think "by the end of the afternoon" might sound more natural than "before the afternoon ends." For example:
(In this case, by the end of the afternoon essentially means the same as by the end of the workday, or before you go home today.)
If the appliance store was going to make a home delivery, I'd expect them to say something more like:
As for drinks, I really don't expect I'd ever suggest to my friends:
Instead, I might say one of these:
I'll offer one more example. Say we're planning a picnic; I might say:
seems more natural than:
1 (or dinnertime, but that's another question...)
What the fragments "before the afternoon arrives" and "before the afternoon ends" do is make afternoon into something which can act, which can do something.
While day can break and night can fall, morning and afternoon don't generally do anything; they just are. Consequently they don't end or arrive, and one says "before the end of the afternoon", or "before the end of the morning".
Because the use of before sets a deadline, it's usual to align it with the end of a period, not its beginning, so one wouldn't generally say "before the start of the afternoon", but "before the end of lunch".
Following Kris's comment, perhaps the question needs to be explicitly answered: Before the afternoon arrives or ends is grammatical but is certainly not idiomatic. Using those expressions is not generally to be recommended.
Yes. The afternoon arrives and ends at certain times, so this is a way to put it. Whether it's the best way is debatable.
Note, though, that those times will be different in different contexts, but will almsot always be specifically-defined.
"Before noon" is non-specific and therefore often meaning any afternoon, but not always. It's also less specific in terms of the timeframe, not implying any hurry. The phrase "before the afternoon arrives" carries some urgency. Even though it doesn't specify exactly which day; could refer to any day; it specifies it's the day, adding more impact or urgency.
This is from my understanding of the phrase in common usage, not a scholarly appraisal of these related phrases.