Convertimos la caja en una mesa.

Convertimos is the first person plural form of the verb convertir, so it means we convert or we converted, depending on context.

The infinitive is convertir = to convert / to turn into / to transform.

So:

• yo convierto
• tú conviertes
• él/ella convierte
• nosotros convertimos
• vosotros convertís
• ellos convierten

It can mean either one.

In Spanish, convertimos is ambiguous because it can be:

• present tense: we convert
• preterite: we converted

Only the context tells you which one is meant.

For example:

• Cada año convertimos la caja en una mesa. = Every year we convert the box into a table.
• Ayer convertimos la caja en una mesa. = Yesterday we converted the box into a table.

So if you see convertimos by itself, both readings are possible.

Because the normal pattern with convertir is:

convertir algo en algo

That means:

• to convert something into something
• to turn something into something

So:

• Convertimos la caja en una mesa. Literally: We turned the box into a table.

This is just the standard construction in Spanish. English uses into, while Spanish usually uses en after convertir.

Other examples:

• Convirtieron el garaje en una habitación. = They turned the garage into a room.
• La crisis convirtió el problema en un desastre. = The crisis turned the problem into a disaster.

This is about the articles.

• la caja = the box
• una mesa = a table

Spanish is being specific about the starting object: the box.
But the result is introduced as a table, not necessarily the table.

So the sentence presents:

• a known or specific object being changed: la caja
• a new identity/result: una mesa

You could also say la mesa in a different context if the table were already identified:

• Convertimos la caja en la mesa que necesitábamos.

But in the basic sentence, una mesa sounds natural.

Because Spanish often drops subject pronouns when the verb already makes the subject clear.

Convertimos already tells you the subject is we.

So:

• Convertimos la caja en una mesa. and
• Nosotros convertimos la caja en una mesa.

both work.

Adding nosotros usually gives extra emphasis, contrast, or clarity:

• Nosotros convertimos la caja en una mesa, no ellos. = We turned the box into a table, not them.

The direct object is la caja.

That is the thing being acted on:

• We convert the box

The phrase en una mesa tells you the result of the change:

• into a table

So the structure is:

Convertimos + la caja + en una mesa
verb + direct object + result phrase

A good way to think of it is:

• What did we convert? → la caja
• What did we convert it into? → una mesa

Sometimes, but not with exactly the same meaning.

Convertir ... en ... emphasizes transformation:

• one thing changes into another

Hacer usually means to make or to do, and it does not always express that same idea of changing one object into a different form.

For example:

• Hicimos una mesa con la caja = We made a table from the box
• Convertimos la caja en una mesa = We turned the box into a table

These are similar, but convertir focuses more on the box being transformed.

So in this sentence, convertir is a very natural choice.

This sentence uses convertir, which is transitive: it takes a direct object.

• Convertimos la caja en una mesa. = We turned the box into a table.

Here, la caja is the thing being changed.

Convertirse is reflexive/intransitive and means to become / to turn into:

• La caja se convirtió en una mesa. = The box turned into / became a table.

So:

• convertir algo en algo = to turn something into something
• convertirse en algo = to become / turn into something

That is a very common contrast in Spanish.

Because convertir is a stem-changing verb, but the stem change does not happen in every form.

In the present tense:

• yo convierto
• tú conviertes
• él/ella convierte
• nosotros convertimos
• vosotros convertís
• ellos convierten

The change e → ie happens in most forms, but not in nosotros or vosotros.

So:

• convierto
• conviertes
• convierte but
• convertimos
• convertís

That is why you see convertimos, not conviertimos.

A Spain Spanish pronunciation would be approximately:

com-behr-TEE-mos la KA-ha en OO-na ME-sa

A few useful points:

• v and b sound very similar in Spanish
• z does not appear here, but in much of Spain it would sound like th in words that have z or soft c
• j in caja is a strong throaty sound, like the ch in Scottish loch
• the stress in convertimos falls on ti: con-ver-TI-mos

And caja is stressed on the first syllable:

• CA-ja

Yes, in many contexts English might translate it that way.

Spanish simple present often covers ideas that English may express with:

• we convert
• we are converting
• sometimes even a general or habitual action

So if the context is about something happening right now, English might naturally say:

• We’re turning the box into a table.

If the context is a finished past event, then:

• We turned the box into a table.

So the best English translation depends on the surrounding context, not only on the isolated Spanish sentence.