Have you ever stopped to wonder why we say perféct when we mean the verb and pérfect when we mean the noun or adjective? There’s a whole set of such contrasts, called minimal pairs, in which the verbal stress is on the final syllable and the nominal stress–meaning either that of a noun or an adjective–is on the initial. Think of prodúce vs. próduce, conflíct vs. cónflict, insért vs. ínsert, frequént vs. fréquent, and so on. Most of the members of such pairs each consist of two syllables, so that the contrast between final and initial stressed syllable holds.
Although, loosely speaking, ACCENT and STRESS can refer to the same thing, in the parlance of linguistics, strictly speaking, STRESS is the term used to mean the emphasis placed on the sound or syllable spoken most forcefully in a word or phrase. This is the meaning foregrounded in the mimetic joke about “putting the stress on the wrong sylláble.” The syllable that has that kind of emphasis in a word is called STRESSED, and syllables that don’t are called UNSTRESSED. In English, words can have both a primary and a secondary stress–several in the case of secondary, but only ONE primary stress.
However, there are also verb/noun pairs where the stress falls on a different syllable, and each contrasting word consists of more than two syllables, like envélop vs. énvelope, interchánge vs. ínterchange, reprimánd vs. réprimand, and so on. Even though in some of these cases the stress need not contrast–réprimand with initial stress does double duty for many speakers as both a verb and a noun–the important and unalterable fact is that no matter how many syllables the word has, if there is a contrast at all, the stress in the verbal form will be NON-INITIAL, i.e. be on one or more syllables closer to the end than in that of the nominal form. Moreover, and just as importantly, THE REVERSE IS NEVER TRUE: there are no English verb/noun pairs which contrast by having an initial stress in the verbal form and a non-initial in the nominal form. Isn’t that curious?
The same invariable relationship between verbal and nominal holds for cases where the noun is an obvious product of NOMINALIZATION, i.e. where a verb phrase is turned into a noun, thus fill ín (“John filled in for Mary”) vs. fíll-in (“John was a fill-in for Mary”), or rent a cár (“You can rent a car at the airport”) vs. rént-a-car (“There’s a rent-a-car at the airport”). Whereas in the first member of each of these pairs the primary stress falls on the final syllable, its nominalized counterpart has primary stress on the initial syllable. We say, therefore, that the stress in the nominal form has SHIFTED in comparison to the verbal form from which it has been DERIVED.
We should always ask ourselves WHY? in such cases. Here the answer lies in the special kind of parallelism–called an ISOMORPHISM–between, on the one hand, the RELATIONAL VALUE of the verb as a category and the RELATIONAL VALUE of the noun (more accurately: the nominal form) as a category; and, on the other hand, the corresponding RELATIONAL VALUES of the positions of stress in each category. Now, what distinguishes verbs from nouns is that every verb NECESSARILY MAKES REFERENCE TO TIME, whereas a noun DOES NOT. When a category in language is defined vis-à-vis another category by necessary reference vs. non-necessary reference to some feature of sound or sense, the first category is characterized as MARKED, and the second as UNMARKED. “Marked” here means “relatively restricted in (conceptual) scope,” and “unmarked” means “relatively unrestricted in (conceptual) scope.” This meaning translates the opposition of marked vs. unmarked into such values as “uncommon” vs. “common,” “atypical” vs. “typical,” and so on.
As with all linguistic oppositions, the same is true when it comes to the relational value of the position of stress in the words and phrases we’ve been considering. In English, for historical reasons, the initial syllable has come to be the “typical” or “unrestricted” syllable as far as bearing the primary stress is concerned. In other words, stress on the initial is UNMARKED, whereas stress on non-initial syllables is MARKED.
It is this PARALLELISM OF FORM between grammatical category and position of stress that accounts for and answers the question, WHY THE DIFFERENCE IN STRESS: non-initial stress correlates with verbal (i.e., non-nominal) forms, and initial stress correlates with non-verbal (i.e. nominal forms). More significantly: marked category goes with marked stress position, unmarked category with unmarked stress position.
At its core, language always displays such isomorphisms. It is these correlations of value that enable linguistic facts to cohere and to form a structure, to be learned by new generations of speakers, and to be perpetuated in the history of a language.