What, then, is the difference between the and that? To ascertain this, let us inquire, in what cases the is employed, and whether that can be substituted in its stead.
The word the is employed,
1st, When we express an object of eminence or notoriety, or the only one of a kind in which we are interested, as, “the king,” when we mean “the king of England.” “He was concerned in bringing about the revolution,” when we mean the revolution in this country. “Virgil copied the Grecian bard,” or “Homer.” “I am going to the city,” when I mean “London.” In none of these cases can we substitute that for the, without laying a particular emphasis on the subject, and implying that its character is there described in contradistinction to some other of the same species. Thus, “he was concerned in that revolution, which was accomplished by the English barons.” “He copied that Grecian bard, who disputes the claim of antiquity with Homer.”
2dly, We employ it in expressing objects of repeated perception, or subjects of previous conversation. I borrow an example from Harris. If I see, for the first time, a man with a long beard, I say, “there goes a man with a long beard.” If I see him again, I say, “there goes the man with the long beard.” Were the word that substituted for the, the same observation would be applicable as in the preceding examples.
3dly, Mr. Harris has said, that the article a is used to express objects of primary perception, and the employed to denote those only of secondary perception. This opinion is controverted by the author of the article Grammar in the Encyclopedia Britannica, Ed. 3d. who gives the following example to disprove its truth. “I am in company, and finding the room warm, I say to the servant, Request the gentleman in the window seat (to whom I am an entire stranger) to draw down the sash.” The example is apposite, and is sufficient to overturn the hypothesis of Mr. Harris. There can be no question but the is frequently employed to denote objects of primary perception; and merely particularizes, by some discriminating circumstance, an individual whose character, person, or distinctive qualities, were previously unknown. In the example now quoted, that may be substituted for the, if we say, “who is in the window seat.”
4thly, The definite article is used to distinguish the explicative from the determinative sense. In the former case it is rarely employed: in the latter it should never be omitted, unless when something still more definite supplies its place. “Man, who is born of a woman, is of few days, and full of trouble.” Here the relative clause is explicative, and not restrictive; all men being “born of a woman;” the definite article therefore is not employed. “The man” would imply that all men are not thus born; and would confine the predicating clause to those who are. In the latter sense, that may, without any alteration in the phraseology, be substituted for the article; for the man, and that man, are in this instance equivalent.
5thly, The definite article is often used to denote the measure of excess. “The more you study, the more learned you will become;” that is, “by how much the more you study, by so much the more learned you will become.” “The wiser, the better;” “that (by that) wiser, that (by that) better.” There also that and the may be considered as equivalent; and the Latins accordingly said “eo melior.”
From the preceding examples and observations it must appear, that the definite article, and the word that, though not strictly synonymous, are words nearly of the same import.
Their difference seems to be,
1st, That the article the, like a, must have a substantive conjoined with it; whereas that, like one, may have it understood. Speaking of books, I may select one and say, “give me that,” but not “give me the;” “give me one,” but not “give me a.” Here the analogy holds between a and one, the and that.