They both have exactly the same function - to indicate that the object or person being modified is definitely known to us. "The car is blue." "He started the car." In both cases, it is assumed that we know about the car being mentioned, usually from the context. The only difference here is that in the first sentence, "the" is used with the subject; in the second, "car" is the predicate.

How to use "the"  Denoting one or more people or things already mentioned or assumed to be common knowledge - what's the matter? - call the doctor - the phone rang Used to refer to a person, place, or thing that is unique - the Queen - the Mona Lisa - the Nile Denoting a disease or affliction - I've got the flu (with a unit of time) The present; the current - dish of the day - man of the moment Used instead of a possessive to refer to someone with whom the speaker or person addressed is associated - I'm meeting the boss - how's the family? Used with a surname to refer to a family or married couple - the Johnsons were not wealthy Used before the surname of the chief of a Scottish or Irish clan - the O'Donoghue Used to point forward to a following qualifying or defining clause or phrase - the fuss that he made of her - the top of a bus - I have done the best I could (chiefly with rulers and family members with the same name) Used after a name to qualify it - George the Sixth - Edward the Confessor - Jack the Ripper Used to make a generalized reference to something rather than identifying a particular instance - he taught himself to play the violin - worry about the future Used with a singular noun to indicate that it represents a whole species or class - they placed the African elephant on their endangered list Used with an adjective to refer to those people who are of the type described - the unemployed Used with an adjective to refer to something of the class or quality described - they are trying to accomplish the impossible Used with the name of a unit to state a rate - they can do 120 miles to the gallon Enough of (a particular thing) - he hoped to publish monthly, if only he could find the money (pronounced stressing “the”) Used to indicate that someone or something is the best known or most important of that name or type - he was the hot young piano prospect in jazz Used adverbially with comparatives to indicate how one amount or degree of something varies in relation to another - the more she thought about it, the more devastating it became Used to emphasize the amount or degree to which something is affected - commodities made all the more desirable by their rarity