Which of the following are correct?
- Will let you know in the afternoon.
- Will let you know afternoon.
- Will let you know after noon.
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.Sign up to join this community
Will let you know in the afternoon.
This sentence is correct. The definite article defines which afternoon you're talking about, therefore there's nothing missing.
Will let you know afternoon.
Here the definition of which afternoon you have in mind is missing. Possible ways of correcting the sentence:
Will let you know [ this / tomorrow / next / Saturday ] afternoon.
Will let you know after noon.
This sentence is technically correct, but the meaning is different. Afternoon is defined as the time from noon (12:00) to 18:00, whereas when you say after noon, you imply right after noon = right after 12:00. The time is limited closely to after 12:00. Similar to as when you say:
I will do this after lunch.
Here you say that after you finish eating your lunch, you will go on doing the promised thing. The same would apply to "after noon."
1 & 3 are acceptable.
2 is incorrect because afternoon is not the object of the verb "know". For it to be grammatically correct, the meaning would have to be "I will teach you what afternoon is like." Therefore you need a preposition to specify the relationship between afternoon and the rest of the sentence.