Take a word that names the one that is more than all the others: tallest, youngest, greatest. Because only one thing can be the tallest, the small word before it is the. The same is true of same, which names the one that is no different.
The lab holds 405 lines from the novel, each with one of four words in the middle: an -est word, same, next or most before an adjective. The scan reads the word directly before it.
Three words that follow the rule
Counted on this page, the rule is right on 127 of the 142 lines with an -est word: 93 with the and 34 with a possessive, such as her or our. For same it is right on 69 of 69. That is the strongest rule in this course.
Next is weaker: 49 of 72, or 68.1%. The reason is not a different meaning but a different use. Next week is a time, and it stands alone; next to names a place; and in when they next met the word is not before a noun at all.
What the 15 are
The 15 lines that do not fit are of four kinds. In the two youngest and the best and safest, the is there and the scan, which reads one word, does not see it. In it would be wisest there is no noun to point to. In dearest Lizzy the word is a name. So the rule is nearly right, and the misses are lines it cannot read.
The word that is two words
Choose most. Now the rule is right on 40 of 122 lines, which is 32.8%. Look at the column: the small word before most is the 37 times, a possessive 3 times, and a or an 45 times. The other 37 have no small word at all.
The 45 are the lesson. A most delightful evening and a most excellent ball do not name the best evening or the best ball. Most means very, and the small word is the one you use before any adjective and noun. Fifteen of the 37 others follow was, been, am, is, be or seemed: she was most kind. There too most means very.
What to do with it
Ask what the word is doing. If it picks the one from a group, use the. If it means very, use the small word you would use without it. The page can count the word before, but it cannot read meaning, so the second step is yours.
The lines are from 1813. The meaning of very for most is old and formal, and is less common now. The lab cannot say how much less.