I'll send this to our Spanish expert, SraJMcGin.
(I've lost this once already!)
1. If the word ends in a vowel (a, e, i, o, u) or the consonants n or s, and there is no written accent mark, you will hear the next-to-last syllable as in muchacho.
2. If you want to hear the last syllable, there must be a written accent mark, as in condición, pidió
3. if the word ends in any consonant EXCEPT n or s, and there is no written accent mark, you will hear the last syllable. hospital / condición
4. if you want to hear the next-to-last syllable, there must be a written accent mark. (too tired to think of an example!)
Then every interrogative (question word) must have an accent: qué, por qué, quién, cuál, etc.
Many short words have accent marks to explain the difference. de (preposition), dé (verb), si (if) but sí (yes)
Then with dipthongs (a strong vowel with a weak vowel, strong ones being a, e, o and weak vowels being u and i. (remember "u" and "i" are weak!) If there is no written accent mark, you will hear the strong vowel as in oigo but if you want to hear the weak vowel, you must put an accent mark on it as in oí.
This is essentially it. Just learn the accent marks with the word.
You did not answer the question completely though. So for the phrase "estoy bebiendolos" where is the accent placed? Don't you have to add an accent when the direct object is attached at the end of a progressive tense?
Yes. Did you not see my answer?
Oh dear, I see that I lost that as well!