Stressed "the" and unstressed "the" have different implications for meter. (And meaning.) "The" has two pronunciations in English. "Thee" and "thuh". The former is stressed, the latter not. While part of it follows the same pattern as a/an governed by by the initial sound, some of the rules are complicated. "Thee" is also used for emphasis as a demonstrative.
"Give me thuh cat toy." (Some ordinary toy.)
"Give me thee cat toy." (The one with special powers.)
The pattern of the articles the and a/an being affected by the sound of the following word dates to Old English. The use of the stressed form as an emphatic does as well. It probably goes back further. There are similar traits in the other Germanic languages. German routinely reduces its definite article die ("dee"), to approximately "duh", and its indefinite article ein ("ayn") to approximately "uhn", as well. Except when emphasized. We even have traces of emphatic "a" in English which is now completely archaic -- except still irresistible in "an historic moment". That probably counts as a fixed expression now. But etymologically speaking, when someone says that they're saying "one historic moment" and emphasizing its distinctiveness. Germans would say "ein historischer Moment" and it's a safe bet they'd say ein (one) and not the usual "uhn" (a).
Sure, but the article doesn't attempt to classify the stress, and the stress is often flexed as a convenience to help the meter or to avoid hard -to-pronounce double vowels, not to change emphasis.
"I don't what that one; I want thee other one. Thuh yellow one, not thee orange one."
"Give me thuh cat toy." (Some ordinary toy.)
"Give me thee cat toy." (The one with special powers.)