Mi tía tiene elegidos un collar sencillo y una pulsera bonita para la cena de mañana.

Because tener + past participle and haber + past participle do not mean exactly the same thing in Spanish.

• Ha elegido = she chose / she has chosen.
  This focuses on the action itself.
• Tiene elegidos = she already has them chosen.
  This focuses on the resulting state: the choice has been made, and now those items are the selected ones.

So in this sentence, Mi tía tiene elegidos... suggests something like My aunt already has a simple necklace and a pretty bracelet picked out for tomorrow’s dinner.

Also, unlike English have chosen, Spanish tener + participle is not a normal perfect tense. It is a separate structure with its own meaning.

Because in this structure, the participle agrees with the direct object, not with tía and not with tiene.

The direct object is:

• un collar sencillo
• una pulsera bonita

Together, those make a plural direct object, so the participle must also be plural: elegidos.

If there were only one masculine item, you would use elegido:

• Mi tía tiene elegido un collar.

If there were only one feminine item, you would use elegida:

• Mi tía tiene elegida una pulsera.

Because Spanish uses the masculine plural when referring to a mixed group containing at least one masculine noun.

Here you have:

• un collar = masculine
• una pulsera = feminine

A mixed pair becomes masculine plural, so the participle is elegidos.

If both nouns were feminine, you would use elegidas:

• Mi tía tiene elegidas una falda y una pulsera.

Because adjectives in Spanish agree with the noun they describe in gender and number.

• collar is masculine singular → sencillo
• pulsera is feminine singular → bonita

So:

• un collar sencillo
• una pulsera bonita

This is normal adjective agreement in Spanish.

Because Spanish normally uses a possessive adjective directly before the noun:

• mi tía
• tu hermano
• su casa

You do not usually say la mi tía in standard modern Spanish.

So mi tía simply means my aunt.

Because the most neutral and common position for descriptive adjectives in Spanish is after the noun.

So:

• un collar sencillo
• una pulsera bonita

This is the standard, straightforward order.

Putting an adjective before the noun is sometimes possible, but it often sounds more literary, emotional, or changes the nuance slightly.

Yes, you can, but the effect is different.

• un collar sencillo sounds neutral and descriptive: it is a simple necklace.
• un sencillo collar sounds more stylistic or emphatic, closer to a mere/simple necklace.

Similarly:

• una pulsera bonita = a pretty bracelet
• una bonita pulsera = also possible, but more expressive or literary in tone

For a learner, the safest default is to put these adjectives after the noun unless you know the nuance you want.

It means for tomorrow’s dinner.

Breaking it down:

• para = for
• la cena = dinner / supper
• de mañana = of tomorrow / tomorrow’s

So the whole phrase tells us the purpose of the items: they are chosen for that event.

Here it clearly means tomorrow, not morning.

That is because the phrase is la cena de mañana:

• cena = dinner
• so la cena de mañana = tomorrow’s dinner

If mañana meant morning, it would not fit naturally with cena.

Because para is used for purpose, intended use, or destination.

Here, the necklace and bracelet are chosen for a specific occasion:

• para la cena de mañana

That means for tomorrow’s dinner in the sense of to wear at / intended for that dinner.

Using por would not express that idea correctly.

Yes, it is natural, especially when Spanish wants to emphasize the idea of already having something selected.

This pattern is common:

• Tengo preparados los documentos.
• Tiene decididos varios cambios.
• Tenemos elegidas las fechas.

So tiene elegidos works well here.

That said, many speakers might also choose the simpler:

• Mi tía ha elegido un collar sencillo y una pulsera bonita para la cena de mañana.

That version is also correct, but it focuses more on the completed action than on the resulting state.

Yes, a little.

Ha elegido is more general and probably more common in everyday speech when you simply want to say someone chose something.

Tiene elegidos is perfectly correct, but it sounds more like:

• the decision is already made
• the items are already picked out
• the preparation is complete

So it is not strange, but it is slightly more specific in meaning.

No, not necessarily.

It only says that she has chosen those items for tomorrow’s dinner. It does not say she is wearing them now.

If you wanted to say she is wearing them, you would need a different verb, such as:

• llevar
• ponerse

For example:

• Mi tía llevará un collar sencillo y una pulsera bonita a la cena de mañana.
• Mi tía se pondrá un collar sencillo y una pulsera bonita para la cena de mañana.

Yes, absolutely.

In fact, has picked out is often a very natural translation for tiene elegidos because it captures the idea that the items have already been selected and set aside.

So possible English translations include:

• My aunt has chosen a simple necklace and a pretty bracelet for tomorrow’s dinner.
• My aunt has picked out a simple necklace and a pretty bracelet for tomorrow’s dinner.
• My aunt already has a simple necklace and a pretty bracelet chosen for tomorrow’s dinner.

The exact best translation depends on how much you want to show the nuance of tiene elegidos.