Cuando cambiamos la cama de sitio, vimos que el somier estaba un poco suelto y que el cabecero tenía polvo.

Because cambiamos is the preterite, which is used for a completed action: we moved/changed the bed’s position.

Here, the sentence is telling a short sequence of events:

1. We moved the bed
2. We noticed something

So cambiamos fits better than cambiábamos, which would suggest an ongoing or repeated action in the past, like while we were moving the bed in a more descriptive sense.

If you wanted to emphasize the action in progress, you might say something like:

• Cuando estábamos cambiando la cama de sitio...

But in the original sentence, the movement is treated as a finished event.

This is a very common Spanish past-tense pattern.

• vimos = preterite, for the main event that happened and moved the story forward
• estaba / tenía = imperfect, for background states or descriptions

So the sentence works like this:

• vimos = we saw/noticed
• el somier estaba un poco suelto = the bed base was a bit loose
• el cabecero tenía polvo = the headboard had dust / was dusty

The idea is:

• the act of noticing happened at a specific moment
• the looseness and the dust were existing conditions at that time

That is exactly the kind of contrast Spanish often makes with preterite + imperfect.

It means to move the bed to a different place or to change where the bed is.

This is a very natural expression in Spanish:

• cambiar algo de sitio = to move something to another spot/place

Examples:

• Cambiamos el sofá de sitio. = We moved the sofa.
• Voy a cambiar la mesa de sitio. = I’m going to move the table.

A literal English translation like change the bed from place sounds odd, but in Spanish it is idiomatic.

Yes. Mover la cama is perfectly natural and often simpler.

Difference in feel:

• mover la cama = move the bed
• cambiar la cama de sitio = move the bed to a different place / change its position

The second one highlights the idea that the object ended up somewhere else, not just that it was physically shifted.

So both work, but cambiar ... de sitio is a very common everyday expression.

El somier is the bed base, the structure under the mattress that supports it.

In Spain, somier is a normal everyday word. Depending on the type of bed, it may refer to:

• a slatted base
• a metal frame with springs
• the part under the mattress in general

English speakers often do not have one single everyday word that matches it exactly, so translations vary: bed base, slatted base, bed frame, or sometimes box spring depending on context.

Here suelto means loose.

So:

• el somier estaba un poco suelto = the bed base was a little loose

This is not the meaning free or single/individual, even though suelto can mean those things in other contexts.

For example:

• un tornillo suelto = a loose screw
• el cable está suelto = the cable is loose

So in this sentence it means something was not firmly fixed.

Because suelto agrees with el somier, which is masculine singular.

• el somier → suelto
• la cama → suelta

Adjectives usually agree in gender and number with the noun they describe.

So:

• el somier estaba suelto
• la pata estaba suelta
• los tornillos estaban sueltos

Because Spanish often repeats que before each new subordinate clause, especially in careful or natural speech.

So:

• vimos que el somier estaba un poco suelto y que el cabecero tenía polvo

means:

• we saw that the bed base was a bit loose and that the headboard had dust

In English, the second that is often optional. In Spanish, repeating que sounds very normal and is often clearer.

You may also hear people omit the second que in some contexts, but repeating it is usually the safest and most standard choice.

El cabecero is the headboard of a bed.

It comes from cabeza (head), because it is the part at the head end of the bed.

In bedroom vocabulary:

• la cama = bed
• el colchón = mattress
• el somier = bed base
• el cabecero = headboard

Both are possible, but they are slightly different.

• tenía polvo = it had dust / there was dust on it
• estaba polvoriento = it was dusty

Tener polvo is very common in everyday speech when talking about furniture or objects that have collected dust.

Examples:

• La estantería tenía polvo.
• La mesa tenía mucho polvo.

Estar polvoriento/a is also correct, but it can sound a little more descriptive or less everyday in some situations.

So the original sentence sounds very natural and conversational.

Because polvo here is being used as an uncountable noun in a general sense: dust.

So Spanish normally says:

• tenía polvo = it had dust on it

Using el polvo would usually refer to a specific dust already identified in the conversation, which is not the point here.

Compare:

• La mesa tenía polvo. = The table was dusty / had dust on it.
• Quité el polvo de la mesa. = I removed the dust from the table.

Because Spanish uses the definite article more often than English does.

In English, we sometimes say:

• we moved bed
• headboard had dust

But Spanish normally needs the article with these nouns:

• la cama
• el somier
• el cabecero

This is especially common when talking about familiar, specific objects in a situation. Since the bed and its parts are clearly identifiable, the definite article is natural.

Because Spanish often leaves out subject pronouns when the verb ending already makes the subject clear.

• cambiamos = we changed/moved
• vimos = we saw

The -amos and -imos endings already tell you the subject is nosotros.

Including nosotros is possible, but usually only for emphasis, contrast, or clarity:

• Nosotros cambiamos la cama, no ellos.

Without that kind of emphasis, omitting the pronoun is more natural.

Mostly when.

In this sentence, cuando cambiamos la cama de sitio means when we moved the bed.

Because cambiamos is in the preterite, it presents the action as a completed event, so when is the best fit.

If the sentence used an imperfect or progressive form, it could feel more like while:

• Cuando estábamos cambiando la cama de sitio... = While we were moving the bed...

So cuando can cover both ideas depending on the tense and context.

Yes. A few natural alternatives are:

• Cuando movimos la cama, vimos que el somier estaba un poco suelto y que el cabecero tenía polvo.
• Al cambiar la cama de sitio, vimos que el somier estaba un poco suelto y que el cabecero tenía polvo.
• Al mover la cama, vimos que el somier estaba un poco suelto y que el cabecero tenía polvo.

They are all natural, but the original sounds very everyday and idiomatic in Spain.

• cuando... = when...
• al + infinitive = on/upon doing something

So Al cambiar la cama de sitio... means roughly When/On moving the bed...