One could say:
The students in Mr Smith's art class were at their easels, looking at the sunset.
In that case, the sunset is the object of scrutiny, and it refers to the entire western sky as it is lit up by the setting sun.
But when it is used to indicate a time of day, the idiom is "before sunset", "after sunset", "at sunset".
EDIT:
CopperKettle's examples on the use of "the" with a proper noun are good ones. Here are some others in the same vein; perhaps we can extract the essence from them to show when it is appropriate to use "the" with the proper noun.
I do not know you any longer! What has happened to the gentle Henry Jekyll, the Henry Jekyll with the wry sense of humor and a fondness for good port?
Get up off your ass, Jones, and get back on that horse! Where's the do-or-die Jones, the gung-ho Jones, the let-me-at-them Jones who volunteered for this mission??? I don't like this new sissy Jones who is afraid to ride on a pony just because it's a little skittish.
Now coming to the question - Recently i looked up in the internet as
to: when is a definite article used ? It says - "We use the definite
article in front of a noun when we believe 'the reader' knows exactly
what we are referring to."
I think this statement is a general statement about the use of
definite article. My first question, Am i right to say that this is a
general statement ?
Yes, you are right. "The" can serve a number of specific grammatical functions that are not covered by this general statement. For instance, in the phrase "the more, the merrier", the matching instances of "the" have nothing to do with the reader's knowledge about what is being discussed. They're simply part of a fixed structure.
Second question is, What if we use 'readers' in place of 'the reader',
will there be a change in meaning then ?
There will be little or no change in meaning, but there will be a slight change in style. Using the plural is the unmarked way to make general statements. Using the singular can be more vivid, and also more literary, which may or may not be what you want. It also has an air of instruction to it, indicating that the speaker/writer is about to embark on a discussion that singles out a specific representative of each class being discussed. In cases like yours, it can, but need not, indicate a one-to-one relationship. Sometimes the plural is used to avoid the necessity of a singular third-person pronoun, which would have to be gender-marked ("he" or "she", "his" or "her", etc.) or inanimate ("it", "its", etc.) or enumerate all genders ("he or she", "his or her", etc.). For example, compare:
"Banks are financial institutions where a lender meets a borrower. The lender gives {his, his or her} money to the borrower..."
with this:
"Banks are financial institutions where lenders meet borrowers. Lenders give their money to the borrowers..."
Both have weaknesses and both strengths. The former is more specific: one lender gives money to one borrower. (This may or may not match reality, but if it is what you wanted to convey, it would be more accurate.) However, the latter is concise and avoids the games writers need to play in order to avoid excluding members of a gender.
In sentence (1), 'a lender' expresses just one lender or any lender ?
In sentence (2), 'lenders' expresses more than one lender or any
lender in general ?
In sentence (1), the context and the reader's prior knowledge make it clear that you are talking about any lender. In sentence (2), the context and the reader's prior knowledge make it clear that you are talking about any lender in general. If the writer suspects that context and the reader's prior knowledge are insufficient to make things clear, and clarity on this point is important, he or she should choose another means to clarify things. For instance, if you wanted to clarify that the relationship between a lender and a borrower is one-to-many (a lender may lend to multiple borrowers, but a borrower only borrows from one lender), you might say:
"Banks are financial institutions where a lender finds one or more borrowers..."
Best Answer
We RARELY say "both the X" in English, but it is valid and meaningful. The use of "the" here emphasizes that we are talking about a particular set of X.
I can't think of an example where the meaning would really change between "both X" and "both the X", other than emphasis. The word "both" implies that I've already introduced two of something and I am now referring back to those two things, so with or without "the", I must be referring to the same two things.
There are other words that can take the place of "both" where the meaning could change. Consider "There are 100 men in this prison. All the men are vicious killers." Clearly by "all the men" I mean "all the men in the prison". But if instead I said, "There are 100 men in this prison. All men are vicious killers", now I'm saying that all men everywhere are vicious killers, not just the men in the prison. Which is probably not what I meant.