How to read "o" in words in Portuguese?
Could you tell me, of course, if it's possible (if there is a rule), when "o" is read /u/ in Portuguese words, and when it should be read /o/? For example, the words "compreender", the "o" here is /o/ or /u/?
You're going to have the /u/ sound if the letter o occur in the end of a sillabe. When at the beginning or in the middle, you get the /o/ sound.
The same will happen to the letter "e": /i/ sound in the end /ê/ or /é/ (it depends on the accent) at beginning and middle.
compreender. com.pre.en.der /compreedê/
Usually the more close to the end of a word, the more chance you have it is an u-sound, except when pronounced really clearly. So compreender is definitely with an o-sound, while most words ending on o are pronounced with u at the end. Some sotaques might show different tendencies though.