Why is one sentence structure correct and not the other?

Why is one sentence structure correct and not the other?
 

Both are wrong. There's a mix of different structures.

No se lo compró todavía ("He didn't buy it <for her> yet", with "it" being a specific known item)

"No le compró nada a su tía todavía" or "No le compró nada todavía a su tía" ("He hasn't bought anything yet for his aunt") [First one has a better layout]

It also can mean "...from his aunt"

Right?

Wrong, surely. How can a su tía mean from his aunt? :thinking:

Tu vendes libros.

Te voy a comprar un libro. (me vendes un libro)

No, right: the possible meanings of an "indirect object" in Spanish are MUCH broader than the English indirect object.

"Le compró a su tía" is inherently ambiguous: whether it means "He(she) bought it for his(her) aunt (on her behalf)" or "He(she) bought it from his(her) aunt" is determined from context or by being more explicit.

Apart from what has been said: for me, the use of "todavía" would need the present perfect (ha comprado) instead of simple past (compró). :thinking:

Use of pretérito perfecto compuesto (not present perfect) would be fine following the language model from Spain. In América pretérito simple has also valor perfectivo. "Son las cuatro de la tarde y todavía no almorcé" is perfectly normal in América, and no matter how strange it may sound to Spaniard's ears, I see no reason to use other tense, specially after "Nueva Gramática ..."

It's context which can disambiguate the meaning of "a su tía". The most likely context is "to his aunt", but it's also possible it to be "from his aunt" or "from her common-law wife" (in an albertnobbsy manner). That's the problem with language forums and lack of context.

Fine. Regional use.
Whatever the name you want to give to it (present perfect as it's called in English or simply antepresente) an action that hasn't been accomplished yet, in America, except in Mexico, can be expressed using past tense. Otherwise, it needs either a compound conjugation, or present.

·Todavía no he comido. = Todavía no como. -> I haven't eaten yet.
·Aún no he salido de la escuela. = Aún no salgo de la escuela. -> I haven't come out from school yet.
·¿Todavía no has terminado el informe? = ¿Todavía no terminas el informe? -> Haven't you finished that report yet?
·Juan no ha llegado todavía. = Juan todavía no llega. -> Juan hasn't arrived yet.