Cargamos las cajas en el camión.

Cargamos is the first-person plural form of cargar, so it goes with we.

In this sentence, it can be:

• present indicative: we load
• preterite: we loaded

That happens because for many -ar verbs, the nosotros form is the same in the present and the preterite.

You usually know from context.

For example:

• Ahora cargamos las cajas en el camión. → present
• Ayer cargamos las cajas en el camión. → past

If the sentence appears by itself, both readings are possible.

If you want to make the present action extra clear, Spanish can also use:

• Estamos cargando las cajas en el camión. = We are loading the boxes onto/into the truck.

Spanish often omits subject pronouns because the verb ending already tells you who is doing the action.

• Cargamos already means we load / we loaded

So Nosotros cargamos las cajas en el camión is possible, but it usually adds:

• emphasis
• contrast
• or clarity

For example:

• Nosotros cargamos las cajas, y ellos conducen.

Las is the definite article, so las cajas means the boxes.

It suggests these are specific boxes that the speaker and listener can identify.

Compare:

• las cajas = the boxes
• unas cajas = some boxes
• cajas = boxes in a more general or indefinite sense

Also, cajas is feminine plural, so the article has to match:

• singular: la caja
• plural: las cajas

Yes, en often means in or on, but with movement into a place or onto a vehicle, Spanish often still uses en where English prefers into or onto.

So:

• en el camión can correspond to in the truck, into the truck, or onto the truck, depending on the real-world situation

English is often more specific here than Spanish.

Because Spanish does not contract en + el.

The only common mandatory contractions with el are:

• a + el = al
• de + el = del

So:

• en el camión is correct
• al camión and del camión are contractions
• but enel camión is not possible

Yes, and it is very natural.

There is a useful difference in focus:

• Cargamos las cajas en el camión
  Focuses on the boxes as the thing being moved.

• Cargamos el camión con las cajas
  Focuses on the truck as the thing being loaded.

This is similar to English:

• We loaded the boxes onto the truck
• We loaded the truck with the boxes

Both are correct, but the structure highlights a different element.

The accent mark shows where the stress goes:

• ca-MIÓN

Without the accent mark, Spanish spelling rules would suggest a different stress pattern, so the written accent is needed.

A useful extra note:

• singular: camión
• plural: camiones

In camiones, there is no written accent because the normal stress rules already give the correct pronunciation.

A simple approximation is:

• cajas ≈ KA-has, but with a stronger, raspier h sound
• camión ≈ ka-MYON

More specifically:

• the j in cajas is the Spanish sound heard in Spain, similar to a strong throaty h
• the g in cargamos is a hard g, like in go
• the stress in camión falls on the last syllable

A rough full-sentence pronunciation is:

car-GA-mos las KA-has en el ka-MYON

The given order is the most neutral one:

• Cargamos
  • las cajas
    • en el camión

Spanish word order is fairly flexible, but changes usually add emphasis, topic, or contrast.

For example:

• Las cajas las cargamos en el camión.
  This emphasizes the boxes.
• En el camión cargamos las cajas.
  This highlights where the action happens.

So the original sentence is the best basic version for a learner to start with.