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Ciberdúvidas is a top-shelf online resource for all linguistic elements of the Portuguese language.
Founded by José Mário Costa and (the late) João Carreira Bom over ten years ago (!!), the site has been supported by many organizations over the years.
A consistently fascinating and authoritative source of “…esclarecimento, informação, debate e promoção” “…clarification, information, debate and promotion [editor’s note: i may translate ‘promoção’ here as ‘promulgation’]” of the Portuguese language.
There may be no overstating the usefulness of this resource, especially with regards to question of grammar and proper usage of the Portuguese language. A *lot* of work has gone into this resource over the years and anyone interested in acquiring and refining Portuguese language skills, especially at the higher levels.
In addition to the articles that are already on the site, the operators of the site happily and enthusiastically accept questions from readers, so definitely take them up on that offer!
[edit: it occurs to me that I ought to add this caveat: Ciberdúvidas is primarily focused on European Portuguese, though their advice is certainly quite useful to the student of Brazilian Portguese. Watch out for the alternate (original) Portuguese spellings that continental speakers use; Correcto is the European version of correto, for example. Enjoy!]