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.
Just to be clear: In general, you do not use an article with a proper name in English.
Where you do put "a" before a proper name is when you want to indicate that all you know about the person is his name. If someone you know who is named Bob Smith is selected for the job, then you say, "Bob Smith has been selected." If you read the name in some sort of announcement and have no idea who this person is, then you might say, "Someone named Bob Smith has been selected", or "A Bob Smith has been selected." Often in that case you put the name in quotes. This would also apply if there are several people with the same name and you are not sure which it is. "Who was selected?" "A Bob Smith. But I'm not sure if it's the Bob Smith from the English Department or the Bob Smith from the Math Department."
You almost never use "the" in front of a person's name. If you do it's usually when you are specifying just which of several people with similar names you mean. Like in my example above about the two Bob Smiths. Or if you want to make clear that you mean the famous person by this name. Like, "We got THE Bill Gates to speak at our conference", meaning, the actual famous person of that name, not just someone else with a similar name.
Best Answer
For your sentences:
These stuffs are imported from the United States. (countries with states in their names)
History of Britain can date back to The Medieval. But we write: History of Britain can date back to Medieval Times.
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