The pronunciation of the Portuguese of Portugal - Stress rules and accents

Stress rules

Monosyllabic words have the stress on that one syllable. Example: pais (parents).
If there is an acute accent ('), a circumflex accent (^) or a tilde (~) over a letter, the stress is on that syllable. Examples: útil (useful), país (country), baía (bay).
Exceptions:
- When the word has both a tilde and an acute or circumflex accent, the tilde does not indicate stress. Examples: sótão (attic), bênção (blessing/benediction), órfão (orphan), côvão (fish trap).
- When a tilde is in a syllable before the penultimate syllable, it does not indicate stress. This happens in words with suffixes, e.g. Joãozinho (Johnny), sotãozinho (small attic), irmãmente (sisterly).
Else, if the word ends in a letter a, o or e (after first removing any final s, ns or m), the stress falls on the penultimate (one but last) syllable. Examples: falo, falamos, dia, dias, baia, falam, margem, margens.
All other words bear the stress on the last syllable. Examples: assim, caiu, senti, falei, comer, feliz.

For the same rules described in a different, more complicated way, and with many more examples and special cases, see Livro de Estilo.

Note that the Portuguese words notícia and notícias (message, news) need the accent. Apart from marking stress, it also turns the last group of letters into a single syllable: no-tí-cias. Without the accent, the word would have four syllables, with the last i stressed: *noticias.
Note that there is a difference between Spanish and Portuguese here: in Spanish, the correct pronunciation needs no accent, but the wrong one would. Confer the Spanish names María and Mario, and the corresponding Portuguese names Maria and Mário: same stress pattern in both languages, but different written accents.

To determine the stressed syllable from how a word is written, it is necessary to decide what the syllables of the word are. For example, in the word pais (parents), the ai is supposed to be a single syllable, so the whole word has only one syllable, and there is no question where the stress might fall. To get the a and the i to belong to different syllables, and accent is needed: país (country) has two syllables, the second of which is stressed.
Confer the name Luís, which does need an accent even though normally a final -is is automatically stressed without a written accent. Apparently without the accent, the name would become monosyllabic, the only syllable being a ui diphthong. Cf. also partiu (left), where iu together make a single, stressed syllable, ciúme (jealousy) and ciumoso (jealous).
Contrary to how it works with "pais" and "país", in a word like raiz (root; plural: raízes) the a and i are supposed to belong to two syllables, even without the accent. The z versus s makes the difference here: words ending in z never have a monosyllabic ai diphthong, so raiz (pronounced ráiz) just can't be a Portuguese word. If it existed, it would be spelled rais.
Other letters having this effect are l, m, n and r if they do not start a next syllable, and also nh. Examples: adail, contribuinte, demiurgo, juiz, paul (marsh, pool, from Vulgar Latin padule), retribuirdes, ruim, rainha ^©, tainha, ventoinha, but (because n etc. start a syllable) cafeína, contraí-la, juíza, juízo.
The diphthong ui also doesn't need an accent if it is preceded by a vowel: atraiu, contribuiu, pauis (plural of paul). Cf. pau (stick), one syllable, paul (pool), two syllables, Paulo (man's name), two syllables.

The difference between úi and uí is sometimes relevant for the meaning of words: "fluido" means "fluid" (as a substance), and fluído is the past participle of the verb fluir (stream). Inclui = it includes, incluí = I included.

Accent marks

The acute accent (') and circumflex accent (^) indicate stress, but also denote vowel quality: ' indicates relatively open sounds (that they are open means that they have a relatively low tongue position): /O/, /E/ and /a/, written é, ó, á. The ^ indicates more close (higher tongue position) sounds: /o/, /e/ and /3/, written ô, ê, â. These diacritics are only used where they are necessary to indicate stress. The sound differences occur in other positions too, where the default stress rules make an accent unnecessary. The exact vowel quality is not unambiguously indicated in the spelling in these cases.

The tilde (~) always means nasalisation, but not all nasalised vowels and diphthongs are indicated by a tilde: the m and n are also used for that, and in one case, muito, nasality is not indicated at all.

Because the nasal vowels /e~/, /o~/ and /3~/ are relatively close vowels *), the circumflex, not the acute accent, is put over them to indicate stress when it is not in the default position. Examples: Ângulo (not *ángulo), excelência (not *exceléncia).
But note também, parabém, parabéns, porém and many other words, where ém denotes a nasal diphthong, which is why the acute accent is used.

*) /3/ is relatively close in the sense that it is less open than /a/; yet, it is itself a half-open sound, or in other words, a low-mid sound. /3/ is also relatively open in the sense that it is much opener than the close (high) central /1/.

The spelling ão usually occurs in stressed syllables, except in words like sótão, órgão and coraçãozinho. A lot of verbal forms have this sound in unstressed position: falam, falavam, falaram ^©, falariam, aprendam. Probably to avoid excessive use of accent marks, these words are spelled with -am: falam, not fálão.
(Compare falaram ^© and falarão ^©). They would differ only in stress, if it weren't for the fact that the stress also makes the second letter a very different: this again is the difference between open and half-open a.
So a better example, which really only differs in stress, is partiram ^© (they left, or they had left) vs. partirão ^© (they will leave).

The grave accent (`) used to be used for secondary stress, resulting from accented words which had a suffix attached: fácil - fàcilmente, só - sòzinho, último - ùltimamente. Willis in sections 164/165, pages 256/257 also mentions cortêsmente, pêrazinha, portuguêsmente, but uses frequentemente, not freqüêntemente. But the latest spelling reforms abolished these accents: the correct way to write these words is now facilmente, sozinho, ultimamente, cortesmente, perazinha, portuguesmente, frequentemente.

The à still exists in words like à and às, which are contractions of a a and a as (to the). In these words, the accent indicates a sound difference: the two half-open a's contract to form a single, open /a/. Examples: aquela vs. àquela, likewise àquele, àquilo. This grave accent (`) never indicates stress, so the stress follows the usual rules: àquela.

A sound change like this always occurs when two or more half-open a's (phoneme /3/) coincide, and then this is not indicated by an accent. Example: toda a vida [toDaviD3] ^©, which sounds as if written "todàvida" (but that's not the correct way to write it), and sounds very different from a hypothetical "toda vida" /toD3viD3/.

Other examples: Every day life, o dia-a-dia /udi33di3/ [uDiaDi3]. Face to face, cara a cara /kar33kar3/ [karakar3].

Sometimes three of even four half-open a's merge into a single (then often somewhat long) open a. It happens in a text about writer Margarida Rebelo Pinto for example: foi chamada à atenção,
/foiS3m3d3333te~s3~u~/ [foiS3m3da:te~s3~u~].

And also in an epilogue, by Luís de Sousa Rebelo, to José Saramago's "A jangada de pedra", on page 335: "(...) visão essa apoiada numa observação permanentemente atenta à arbitrariedade e à convenção, (...)",
/3te~t3333rbitrari1dad1i33/ [3te~tarbitrari1DaDia].

This merging of two unstressed a's that are pronounced /33/ into a single [a] even happens when one of the a's is a nasalised /3~/. This combination produces a sound that is otherwise not part of the language, i.e. it doesn't occur as an independent sound, other than as a result of this merging: a nasalised open [a~].
Examples: andorinha ^© phonemically /3~duriJ3/, phonetically also [3~duriJ3] ^© versus a andorinha ^© phonemically /33~duriJ3/, but phonetically [a~duriJ3] ^©.
Stress is important here: "a antiga" does get the open nasalised a, but "a a^ncora", where the second a in the merger was stressed, keeps its half-open nasalised timbre.
More examples: Pela ambição [pela~bis3~u~] ^©
'Na Antena 1' (Antena 1 is a National Portuguese radio network). Here, the difference could even become phonemic: 'antena' /3~ten3/ versus 'a antena' /a~ten3/. So maybe there really is a separate phoneme /a~/ in the Portuguese language?!