The pronunciation of the Portuguese of Portugal

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.

Merging unstressed a

A sound change like this always occurs when two or more unstressed 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 or 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?!

Trema

The diaeresis or trema was formerly used to indicate that the u between a q or g and an e or i is not silent (it normally is, like in Spanish, but unlike in Italian). But according to the latest spelling rules this is no longer done, although it is still usual in Brazil.

Acute and circumflex, open and closed

The effect of the acute and circumflex accents in Portuguese may seem rather unexpected for those familiar with French or Hungarian, because they work more or less the other way around in comparison with those languages. French and Hungarian don't have a variable stress, so the accent marks are not used to indicate stress. But they can indicate vowel quality:

The unaccented e of Hungarian is very similar in sound to the accented é of Portuguese.
The accented é of Hungarian has a sound that is close to that of Portuguese ê. But in Portuguese it is shorter, because in Hungarian the acute accent is also – or rather: mainly – a length marker.
Likewise, an accented Hungarian ó has a sound similar to Portuguese ô, but it is longer in Hungarian. The Portuguese ó resembles an Hungarian a (not o!) without accent.
The Hungarian o without accent is short and has a sound similar to Portuguese ô.
In French too, the vowel quality effect of accent marks is reversed in comparison with Portuguese: an approximation of the French word être (to be) in the Portuguese spelling system could be étrre. French été (summer) would have to be written êtê in Portuguese, or rather etê because there can't be two accents in one and the same Portuguese word.