What is the significance of stressed/unstressed pronouns in Portuguese?

Question

A recurring topic in Portuguese seems to be the use of stressed/unstressed pronouns. However, I am not sure the definitions I have seen make sense.

The only explanation I could find was:

The difference between stressed and unstressed pronouns is that stressed pronouns can act as the subject of a sentence and as its object. On the other hand, unstressed pronouns can never act as the subject.

And then mim, ti, si, are all stressed pronouns. But I do not really see how they can be the subject of a sentence, as when they are used in a passive voice the subject of the sentence shifts away from "mim", "ti", etc

Answer 1

It doesn’t matter if the pronouns are stressed or not for them to act as subject/object. What matters is the case. I guess it’s like in English, where you have different cases for pronouns; subjective case (I, we...) and two oblique cases; objective case (me, us...) and possessive case (my, mine, our/s...).

Mim, ti, si... are ‘pronomes do caso oblíquo’ (oblique cases). Eu, tu, ele/ela... are ‘pronomes do caso reto’ (subjective case). Only ‘pronomes do caso reto’ can act as the subject, ‘oblíquos’ act as objects (‘complementos’).

The stressed pronouns (‘pronomes oblíquos tónicos’) are preceded by a preposition (they are called ‘objects of preposition’), the unstressed ones (‘pronomes oblíquos átonos’) are not preceded by a preposition (they are called ‘objects of verb’).

Answer 2

Here is an answer from Ciberdúvidas:

'A designação de átonos e tónicos refere-se à acentuação. As formas átonas são aquelas que estão junto do verbo, designando-se de pronomes clíticos porque dependem fonologicamente da forma verbal, comportando-se como se fossem uma das suas sílabas (ex.: “Ele entrega-se ao trabalho.”). As formas tónicas têm sílaba tónica e podem estar afastadas do verbo, sendo regidas de preposição no caso de desempenharem a função de complemento (ex: “Para ela, tudo é fácil.”).'

The designation of non-tonic and tonic [stress pronouns] refer to stress (accented syllable). The non-tonic stress ones are ones attached to or next to the verb, and are called "clitics" because they depend phonologically on the form of the verb, behaving as if they were one of its syllables. (For example: Ela entrega-se ao trabalho. She gives herself over to her work.)

[See how it's as if the se were part of the word and the pronoun is unstressed? The verb syllable tre of entrega-se is where the tonic accent falls]

The tonic stress form has a stressed syllable and may not be next to the verb. They are governed by a preposition when they act as a direct/indirect object. (For example, Para ela, tudo é fácil. For her, everything is easy.)

[See how ela contains a stressed syllable in its own right? The e in ela is the tonic stress. Obviously, they can be subjects or objects. You can't append ela to a verb. It can be a subject or an object.]

in Ciberdúvidas da Língua Portuguesa, https://ciberduvidas.iscte-iul.pt/consultorio/perguntas/pronomes-atonos-e-tonicos/8071 [consultado em 01-11-2021]

So, basically, for a learner, is the pronoun stuck to a verb? Then, its pronunciation does not take the tonic stress: "Falar-me dos problemas políticos neste país, me cansa". So, the tonic accent in falar-me falls on the second syllable. Not on the me, where the intonation falls off.

Another way to state this is to say this is that the clitics have falling intonation:

Look at these: (from Priberam)

Ele deu-me um livro.
The tonic stress falls on deu, not on me. So, the stress is falling on me. Whereas: Ele me deu um livro. has tonic stress on me, it cannot fall off.

Dou-te uma caneta. Same as above.