What's the rule for pronouncing similarly-spelt verb conjugations?

Question

Recently, while reading through a book written in extremely formal Portuguese, I came across this line:

Ó, meus amados irmãos, ouvi minhas palavras.

The word in question here is ouvi. Normally when I see this word, it is in this context:

Eu ouvi o que você falou.

I understand the difference in context: the ouvi from the first example is an imperative form (assuming it's conjugated to vós), while the second example is the past-perfect form.

From context, I feel that the first example should be pronounced similar to ouve (as in Ele ouve), and the second example with the emphasis on the last syllable (as in many other past-perfect conjugations with verbs ending in -er and -ir:

bater -> bati
sacudir -> sacudi
correr -> corri
comer -> comi

My question(s) is(are):

• Are these two forms pronounced the same way, or are they pronounced differently?
  • If different, what grammatical or pronunciation rule exists to explain why?

Answer 1

It's the exact same pronunciation for both. That's all there is to it, really. All words ending with i or is and with no accent are stressed in the last syllable.

It’s the same thing with

Ele ouve/bate (present indicative) and ouve/bate (tu, imperative)

In Portugal, but not in Brasil, the first person plural of the present and past-perfect indicative of regular verbs ending -ar are pronounced differently, but it is optional to spell them same or diffently. For instance:

Agora, nós gostamos. Ontem, nós gostamos (ptPT and ptBR) or gostámos (ptPT only)

In Portugal, the ta is pronounced as tâmara in the present, but as tábua in the past-perfect even if you spell it gostamos. In Brasil, on the other hand, they're both pronounced as tâmara.

Answer 2

They're pronounced the same way.

I think that the reason you feel ouvi (vós) is pronounced differently is by analogy with the singular: ouve (tu) or ouça (você) (which are pronounced with stress on the first). But that would go against the general rules of stress in Portuguese, where words ending in i need an accent to be paroxytone (e.g. táxi).

Exceptions like ser aside (és/sê and sois/sede), the forms of the imperative are the same as the ones from the present with the final s dropped and the final s does not affect where the stress falls. So (tu) ouves/ouve (tu) are both paroxytone and (vós) ouvis/ouvi (vós) are both oxytone.

Note that the archaic form of ouvi (vós) already has the stress on the syllable with the i: ouvide (vós).