Mi abuela conserva cartas antiguas en una caja de madera.

Mi is a possessive adjective used before a noun. In modern standard Spanish, it normally replaces the article, so you say mi abuela, not la mi abuela.

Also, mío / mía are usually used in other positions, for example:

• La abuela es mía. = The grandmother is mine.

A useful rule:

• mi = before a singular noun: mi abuela, mi abuelo
• mis = before a plural noun: mis abuelas

Notice that mi does not change for gender.

Conserva is the third-person singular present indicative form of conservar.

It matches the subject mi abuela, which is she:

• yo conservo
• tú conservas
• él/ella conserva

So here, conserva means she keeps / she preserves.

They are similar, but not exactly identical.

• guardar often means to keep, put away, store
• conservar often suggests keeping something over time, sometimes with the idea of preserving it

In this sentence, conserva cartas antiguas sounds natural because old letters are often kept carefully for sentimental or historical reasons.

So:

• Mi abuela guarda cartas antiguas... = also possible
• Mi abuela conserva cartas antiguas... = slightly more like she preserves/keeps them

Spanish often allows a bare plural noun as a direct object when it means something indefinite or non-specific.

So cartas antiguas can mean:

• old letters
• some old letters

If you wanted to make it more explicit, you could also say:

• unas cartas antiguas = some old letters

If you said:

• las cartas antiguas

that would usually mean specific old letters already known from the context.

Two reasons:

1. Position
   In Spanish, most descriptive adjectives usually come after the noun:

   • cartas antiguas
   • caja grande
   • libro interesante

2. Agreement
   The adjective must agree with the noun in gender and number.

Here:

• cartas is feminine plural
• so the adjective must also be feminine plural: antiguas

Compare:

• carta antigua = feminine singular
• cartas antiguas = feminine plural
• libro antiguo = masculine singular
• libros antiguos = masculine plural

You can sometimes put an adjective before the noun, but it often changes the tone or emphasis. Antiguas cartas sounds more literary or stylistically marked than the neutral cartas antiguas.

Because caja is a feminine singular noun.

So it takes the feminine singular indefinite article:

• una caja

Compare:

• un libro
• una carta
• una caja

A lot of nouns ending in -a are feminine, and caja is one of them.

Spanish often expresses material with de + noun:

• una caja de madera = a wooden box / a box made of wood
• una mesa de metal = a metal table
• una botella de plástico = a plastic bottle

This is very common and natural in Spanish. English often uses a material adjective before the noun, but Spanish usually uses de.

Because here madera refers to the material in a general sense, not to a specific wood already identified.

When Spanish names a material in this way, it usually uses:

• de madera
• de metal
• de papel
• de cristal

Without an article.

You would use an article only if you were talking about a specific wood already mentioned or clearly identified in context.

Here en means in and shows location: the place where the letters are kept.

So en una caja de madera tells you where the letters are.

In this sentence, it does not mean movement into the box. It describes the letters as being kept in the box.

Yes, but the given order is the most neutral and natural:

• Mi abuela conserva cartas antiguas en una caja de madera.

That is:

• subject: Mi abuela
• verb: conserva
• object: cartas antiguas
• place phrase: en una caja de madera

Other orders are possible for emphasis, for example:

• En una caja de madera, mi abuela conserva cartas antiguas.
• Mi abuela conserva en una caja de madera cartas antiguas.

These are grammatical, but the original version is the most standard one for everyday use.