Well, Spanish seems to be straight-forward with stress, that is, when and were to lay stress on a given syllable, but I find this is not really so. Sort of like i before e execpt after c in English. There are so many exceptions as to make one almost feel as though disregarding the rule will give them the correct spelling!
Anyway, we learn that Spanish words ending in a vowel or n or s lay their stress on the "penultimate" or second to last syllable. But I find many exceptions that are not accented (since we learn that exeptions must have tildes). Such as sucio, sabio, comercio, negocio and many others.
Is there are formula, or something that has not been expained to me? Or have I just come across a lot of misfit exceptions?
Here is a link to an answer previously given to another member. It will lead you to a couple of SpanishDict reference pages on the subject of your enquiry. Just "click" on the link to find the information. ----> Syllable Emphasis & Accent Marks.
After you have a chance to read it over, if you still have one or more questions, come back, open a new question window and ask your new question.
I hope these pages are a big help.
Two vowels next to each other, absent any tildes, shall be considered a single syllable for purposes of stress assignment.
So, for a made up word "ioioio" there are only three syllables for stress purposes and the second io group would receive the stress (yo-YO-yo)
However, for a made up word "ioioío" would have four syllables, the third i receiving the stress (yo-yo-EE-oh).
I have found it thoroughly consistent. My favorite thing about Spanish is the consistency of the spelling and stress. A new word can immediately be pronounced correctly (or as correctly as my heavy American accent will currently allow).
One way to look at the question of sucio, sabio, comercio, negocio and many others is to consider the last two vowels in words like these as one vowel, as they are in fact pronounced as one (if not pronounced as one, there is usually an accent mark on the stressed vowel of the two).. Keeping this in mind, you can use the second-to last rule as well.
In my opinion, it's best to learn how to lay stress on words just by observing native speakers before reading the rules. Unless there is an accent mark in the word, there really isn't a consistency in the second-to last rule. As the reference explains how stress works in Spanish, my personal experience is that it can be determined naturally and accurately without realizing how the vowels and consonants form a pattern.