In a declarative statement, why would you put a subject pronoun at the end of a sentence or verb phrase?

Question

I was recently doing another Instituto Cervantes activity when I came across the following dialogue:

¿Y no quieres también un pantalón?
No, lo prefiero comprar yo.

And don't you also want a pair of pants?
No, I prefer to buy them myself.

When I initially drafted this, I knew enough Spanish that I didn't bother running these sentences through a machine translator and thought that the response in the dialog above meant "No, I prefer to buy them." Before posting, however, I discovered that "lo prefiero comprar yo" is translated by more than one machine translator as "I prefer to buy it/them myself (in this context, them). So, now my question is what is the difference between:

lo prefiero comprar yo

and

lo prefiero comprar yo mismo

?

What is the purpose of putting the subject pronoun at the end and is this more common in certain Spanish-speaking countries than others? In what types of situations should I put the subject pronoun at the end? Does one carry more force than the other? Is this just a case of "yo mismo" being shortened to "yo" similar to the way some just say "tampoco" instead of "yo tampoco"? Would it be better to translate these two like this:

lo prefiero comprar yo
I prefer to buy it

versus

lo prefiero comprar yo mismo
I prefer to buy it myself

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¿En una frase declarativa, ¿por qué pondría un pronombre sujeto al final de una frase o frase verbal?

Recientemente estaba haciendo otra actividad del Instituto Cervantes cuando me encontré con el siguiente diálogo:

[Véanse arriba en la parte inglesa.]

Cuando elaboré esto inicialmente, sabía español suficientemente bien que no me preocupó de pasar estas frases a través de un traductor automático y pensé que la respuesta en el diálogo de arriba simplemente significaba, "No, I prefer to buy them." Sin embargo, antes de publicar, descubrí que "lo prefiero comprar yo" es traducido por más de un traductor automático como "lo prefiero comprar yo mismo (en este contexto, ellos)". Así que, ahora mi pregunta es ¿cuál es la diferencia entre:

[Véanse arriba en la parte inglesa.]

¿Cuál es el propósito de poner el pronombre sujeto al final y es esto más común en ciertos países hispanohablantes que en otros? ¿En qué tipo de situaciones debería poner el pronombre sujeto al final? ¿Tiene uno más fuerza que el otro? ¿Sería mejor traducir estas dos así:

[Véanse arriba en la parte inglesa.]

Traducción realizada, en parte, con la versión gratuita del traductor www.DeepL.com/Translator.

Answer 1

First, as you may know, automatic translators nowadays do not so much translate but look up people's (presumably correct) translations and try to infer patterns; that is, if a lot of available materials have a final subject pronoun in Spanish rendered into English as a reflexive pronoun, the automatic translator will use that, regardless of the logic.

Second, I tried looking for academic sources but found none specifically relevant to this, so I'll just explain based on what I've read here and there, plus my intuitions as a native speaker.

In any sentence there are two important positions: the beginning and the end. The beginning of the sentence is normally where the speaker introduces the theme or topic, the thing that the rest of the sentence will be about. After the topic comes the comment, a.k.a. rheme or focus, that is, the things that are said about the topic. Often the end of the sentence is the most focal position, where the really new information or unexpected facts are mentioned. All of this is very general; there are other things to be factored in. The topic is "old news", so it might be absent if the speakers has already mentioned it clearly before. The topic might be emphatic or contrastive. The focus can be very "soft", the sentence being rather general with no particular emphasis. That certain Spanish verbs call for verb-subject (VS) order rather than SV order has to do with focus (“Se cayó el jarrón” first says to you something has fallen, and then gives you the really important news: it was the vase!).

In the case of the final subject pronoun (it could be a full noun phrase too), the displacement puts the agent of the action into focus. “Lo prefiero comprar yo” has the old information first (the conversation is about buying pants) and then the news: it's not the other person who will buy the pants, but the speaker.

English cannot dislocate a pronoun in this way, so in the face of such a sentence, it has to resort to other means. With a simpler syntax it could be a cleft sentence, but here I suspect “It's me who prefers to buy them (the pants)” sounds terrible, and “The one who prefers to buy them is me” is ridiculous. The alternative is a reflexive pronoun added for emphasis: “I prefer to buy them myself”.

In Spanish it would be possible to use the emphatic mismo as well, but the pronoun would be dislocated anyway: “Lo prefiero comprar yo mismo”. If the speaker left it in the preverbal position it would sound exactly like English “I myself prefer to buy them”: the meaning would be different.

Answer 2

Although "lo prefiero comprar yo (mismo)" is grammatical, this flows better:

• Prefiero comprarlo yo (mismo).

Indeed, Diccionario Panhispánico de Dudas (DPD) says here, under 3 (d):

Tampoco es normal la anteposición de clíticos con verbos que expresan creencia, temor, deseo, preferencia o conocimiento, como creer, temer, desear, preferir, negar, afirmar, entre otros: Cree haberlo guardado (más normal que Lo cree haber guardado); Prefiero ignorarte (más normal que Te prefiero ignorar); Deseo irme (más normal que Me deseo ir); Negó saberlo (más normal que Lo negó saber), etc. (Placing clitic pronouns before some verbs expressing belief, fear, wish, preference and knowledge is not normal.)

To add to pablodf76's answer, adding "mismo" not only further emphasizes the pronoun in end position but may be interpreted as "by oneself," that is, without the aid of anybody else.