DIACRITICS

The Portuguese language uses acute, circumflex, tilde, grave, dieresis, and cedilla — on some letters:

Á, É, Í, Ó, Ú
Â, Ê, Ô
Ã, Õ
À,
Ü (only in Brazilian spelling)
Ç (never in a begin of a word, before e or i and used in Brazil for spelling Indian words, like: Tomé-Açu)

Acute and circumflex accents

The diacritics "acute accent" and "circumflex accent" are used principally to indicate the stressed syllable of a word. According to detailed rules, the stress diacritic is either written or omitted, this rules depend principally of the position of that syllable (first, second, or third from the end) and on the final letter of the word.
When the stress diacritic (acute or circumflex) is present.

The diacritic acute or circumflex also indicates the vowel’s quality: specifically, "Á", "É", and "Ó" have the so-called "open" sounds, whereas "Â", "Ê", "Ô" have the "closed" sounds. When the vowels "A", "E", "O" carry no diacritics, their sound may be either open or closed, and this attribute cannot always be deduced from the printed word. Thus, for example, seco can be either an adjective ("dry") or a verb ("I dry"); the "E" is "closed" in the first case and "open" in the second. The unmarked vowels "I" and "U" have only one probable sound each, so they may take only the acute accent.

Tilde

The tilde (til) is used to indicate nasalized vowel sounds. This spelling and nasal diphthongs (such as ão, ãe and õe) are a distinctive characteristic of Portuguese. Contrasting the acute and circumflex accents, the tilde does not necessarily indicate stress, and certainly a few words carry both a tilde and a stress diacritic, e.g. ímã ("magnet") and órgão ("organ").

Grave accent

This diacritic is present only over a word-initial “A”, to specify the presence of contracted preposition a ("to", "for", etc.) This grave-marked contraction occurs with only a handful of words, mainly the article a and the different forms of the pronoun aquele ("that"). Thus, a ("to") + a ("the") = à ("to the"); a + aquela = àquela ("to that"); and so on. In all these cases the "À" sounds precisely like "Á" in most dialects.

Dieresis

The dieresis (in Portuguese, trema) is only used in Bazil, but it is increasingly omitted (some newspapers and many people don’t use it). In Portugal, there is no dieresis, in which case the correct pronunciation is word by word.

The dieresis may be used only over the U in the combinations gue, gui, que and qui. These are pronounced [ge], [gi], [ke], [ki] when unmarked; with the diaresis — namely, güe, güi, qüe, qüi — the "U" is pronounced, yielding [gwe], [gwi], [kwe], and [kwi]; e.g. agüentar ("to bear").
The dieresis does not indicate stress, and certainly a word may include other diacritics, such as argüição ("questioning").

Cedilla

The cedilla (cedilha) is used only under the letter "C", only before "A", "O", or "U", and never at the start or at the end of a word: poça ("puddle"), moço ("lad"), açúcar ("sugar"). The mixture "Ç" always sounds [s] as in "sun", even in contexts where the letter "S" would sound [z].