As I read it, in the pronunciation guide here: https://www.portuguesepedia.com/portuguese-word-stress/ words ending in ‘os’ would be stressed on the penultimate syllable. So why does ‘construímos’ need the accent on the ‘i’?
@pvreed, I believe the pronunciation guide just wanted to demonstrate that it’s common for many unaccented words to be stressed on the penultimate syllable (not necessarily because of how they end).
As for the rules governing accents, I recommend this summarized guide: http://publications.europa.eu/code/pt/pt-4100100pt.htm
Regarding construímos, you’ll find a simplified explanation on the guide on why it’s accented (verb form derived from a verb ending in -uir). The more complete explanation is that the two vowels in the middle form a hiatus instead of a diphthong (i.e. they’re pronounced as individual, separate sounds and are part of different syllables), and in those circumstances, a stressed I vowel must have an accent.