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Why is there no article in front of the sentence?

thanks in advance.
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There is no indefinite article with the word "coffee" because "coffee" is an uncountable noun. However, you could say this:

- Afternoon is the time I need a cup of coffee.

The word "afternoon" does not have an article because that word refers generally to a time of day.
In theory, you could also say "the afternoon", but it isn't necessary in your sentence.
Comments  
Thanks for the reply.

But what I wanted to ask is not coffee.

Your answer gives me a new question. I thought 'in front of the sentence' meant

'in the begining of the sentencce.' that is, before 'Afternoon' unlike 'in the afternoon'

Am I wrong?