The means "one X we've talked about or seen before."
It means exactly one specific known/seen X - if there is more than one X, you can't use the unless you qualify the noun with adjectives so we know which one. The X can refer to a plural noun that is an indivisible group logically functioning as one "unit", i.e. the dozen eggs.
It also can be used with nouns that fall into a logical category of "one X that everyone should have seen or heard of before" or "only one possible X in the world."
Introducing a singular noun into a conversation with the indicates the speaker expects the listener to know which one on his/her own.
It's difficult to give general rules on when to use the versus a because the points above heavily depend on variable conditions outside of the conversation.
Walking around Prague I saw the house that was painted red.
Is there a well-known red painted house previously known or seen both the speaker and listener? We then use the to indicate we are referring to it. If a was used, it means a red house was seen, but it wasn't anything special, and it wasn't a specific one known or seen before by the speaker/listener.
English, as you know, is full of exceptions. One of those is generally you do not use the in front of proper nouns that are the names of people.
Technically, you can do this, but you will be making the person sound "legendary" - as though he/she has done something so grand or unique that "one X that everyone should have seen or heard of before" or "only one possible X in the world" applies. So, in that case the Queen Elisabeth could work.
Best Answer
Comparatives and superlatives ("better", "soonest") are adjectives that would not normally take the definite article "the". However, it is grammatical to use the adjective as a noun, where the adjective stands as an ellipsis of a noun phrase. For example:
As in your example, the adjective can also stand for an adverbial phrase that would otherwise include a noun:
There are also set idiomatic phrases like "the X the better", but again these are elliptical phrases that omit redundant information.
I don't think there is any special rule around the use of this other than the sentence should still make sense with the noun omitted. For example:
The least what? It's not clear from context if I mean "the least expensive wine" or "the least fruity wine" or something else. Of course people say things like this all the time, which can require the listener to ask follow-up questions.
There are other cases where the definite article can be used before a comparative or superlative adjective, for example if the adjective has been converted into an ordinary or proper noun, as in someone's name or title. For example, the boxer Muhammad Ali was nicknamed "The Greatest".
More information.