The following was taking from Gramática da Língua Portuguesa by Roberto Melo Mesquita. Before continuing, keep in mind the recent spelling reform did away with the differential accent, such as in the section below on the graphic accent where there used to be a written difference between pára (stop) and para (for). In any event, I will change the color of the font on that section as a reminder of its abandonment.
In any word of two or more syllables, there is always one that is pronounced with greater intensity than the others. Such as:
For the syllable which is pronounced with greater intensity, the name of sílaba tônica (tonic or stressed syllable) is given. The rest receive the title of átonas (atonic, unstressed), as they are pronounced with a lesser intensity. The tonic syllables receive a more intense accent called tônico. Reexamining two of the previous examples, we have:
which is made up of the atonic syllables ‘ja-ca’ and the tonic syllable ‘RÉ’.
which is made up of the tonic syllable ‘GEN’ and the atonic ‘te’.
It is necessary to not confuse the tonic accent with the acento gráfico (graphic accent), which is also called a sinal diacrítico (diacritic sign). The tonic accent signals a greater intensity on a syllable when the word is pronounced, independent of being accentuated graphically or not. The tonic syllable doesn’t always present a graphic accent. See the following:
with the ‘CI’ as the tonic accent
with the ‘CEI’ as the tonic accent
There are words with three syllables or more that have, in addition to atonic and tonic syllables, a secondary accent. This occurs principally with words that are formed from other words. See the following:
with the ‘ca’ as atonic and ‘fé’ as tonic
with the atonic ‘ca’, the subtonic ‘fe’ and the tonic ‘zal’
with the atonic ‘cha’ and the tonic ‘péu’
with the atonic ‘cha’, subtonic ‘peu’, tonic ‘zi’ and atonic ‘nho’
In the Portuguese language, many times it is the tonic accent which permits the identification of the grammatical category of a word or differentiates words within the same category. See the following:
Indication of grammatical category:
Differentiation among words: