is there a rule about when e is pronounced as e as in get and when like the a in gate, ie the difference between certo and cedo. Also is there a rule about the disappearing o at the end of a word?
@lorraineg2011, there’s no rule to make vowels disappear That happens naturally when the endings are unstressed. So, basically, if you’re not hearing an O at the end of a word, it’s because it’s unstressed and its pronunciation is closed (European Portuguese Vowels - Pronunciation Guide | Practice Portuguese). For example, the O of livro might be barely there (unstressed, closed), but the O of avó is clearly articulated (stressed, open).
About the vowel E, based on your examples, you might be especially interested in cases where it falls on stressed syllables, but the pronunciation varies from word to word. I’m not sure that there are clear rules to consider, since a lot of it is guided primarily by etymology, as well as by the need to distinguish between words with identical spelling. For example, for nouns and verbs with the same spelling, the noun will usually be pronounced with a medium E, but the verb form will usually be pronounced with an open E (e.g. um aperto vs. eu aperto). Noun pairs such as sede (headquarters) and sede (thirst) are also only distinguished by the fact that the former has an open E on the first syllable and the latter, a medium E. So, in the end, we have to rely essentially on memorization to navigate all of these multiple possibilities.
Accents help, when we have them: an acute accent (´) always corresponds to an open E, and a circumflex (^) always corresponds to a medium E. Nasal syllables, for example, also consistently have medium Es (e.g. penso, incenso)