This is the 5304th most frequent Portuguese word.
quão
"Quão" means "how" or "to what extent" when used for emphasis in questions or exclamations.
This usage of "quão" is as an interrogative adverb, asking to what extent or degree regarding the height of the building.
Quão alto é aquele prédio?
How tall is that building?
Here, "quão" is used as a relative adverb to express the extent of the beauty of the sunset.
Eu não sabia quão bonito seria o pôr do sol aqui.
I didn't know how beautiful the sunset here would be.
This sentence uses "quão" as an adverb to indicate the measure of the speed at which the work was completed.