Portuguese uses the full twenty-six-letter alphabet, with k, w and y confined to borrowings and names. What makes Portuguese spelling distinctive is not the letters but the diacritics, and every one of them changes the word rather than dressing it up.
The five marks
| mark | example use | what it does |
|---|---|---|
| tilde | on a and o | makes the vowel nasal. This is the big one. |
| acute | on any vowel | marks stress, and opens the vowel |
| circumflex | on a, e, o | marks stress, and closes the vowel |
| grave | on a only | marks a contraction of the preposition with the article |
| cedilla | under c | forces an s sound before a, o and u |
The tilde is the one with no English parallel. It nasalises the vowel underneath it, sending the air through the nose as well as the mouth. Portuguese nasal vowels are not the same as the French ones and are not optional: dropping the nasality produces a different word, or no word at all.
The acute and circumflex both mark stress, and they additionally tell you whether the vowel is open or closed. That is a real distinction in Portuguese and it separates pairs of words that are otherwise spelled identically.
The cedilla does one job: it keeps c sounding like an s in front of a, o and u, where it would otherwise be hard. It never appears before e or i, because there it would be doing nothing.
The Brazilian signature
One pronunciation habit identifies a Brazilian speaker immediately: d and t before an i soften into a palatal sound. So the word for day comes out closer to djia than to dee-a. The same happens with t, which moves towards a ch sound.
European Portuguese does not do this. It does something else instead, reducing unstressed vowels so heavily that words contract, which is why the two varieties sound so different despite sharing a spelling.
Stress, and why the accents are worth learning properly
Portuguese stress is predictable from the spelling: words ending in a vowel, m or s are stressed on the second-to-last syllable, and words ending in most consonants on the last. An accent appears exactly when the word breaks those rules.
That makes the accents genuinely informative rather than decorative. A written accent is telling you something you could not otherwise know, and skipping them in your own writing removes information the reader needs.