Cambiamos el brillo sin que cambie el contraste.

Because sin que is normally followed by the subjunctive.

Here, cambie is the present subjunctive of cambiar. Spanish uses the subjunctive after sin que because the action in that clause is not being presented as a stated fact; it is something being avoided:

• Cambiamos el brillo sin que cambie el contraste.
• We change the brightness without the contrast changing.

So cambia would not fit here; cambie is the expected form.

Sin que means without when it is followed by a full clause with its own verb.

A useful comparison:

• sin + infinitive = without doing something
• sin que + subjunctive = without someone/something doing something

So in this sentence:

• sin que cambie el contraste = without the contrast changing

It introduces a separate action/state that does not happen.

The subject of cambie is el contraste.

So the structure is:

• cambie = changes
• el contraste = the contrast

Literally, this part is:

• without the contrast changing

That is why the verb is singular: el contraste is singular.

Yes, absolutely. That would also be natural Spanish.

• Cambiamos el brillo sin cambiar el contraste.
• We change the brightness without changing the contrast.

The difference is mostly structural:

• sin cambiar el contraste uses an infinitive
• sin que cambie el contraste uses a subordinate clause

A rough nuance:

• sin cambiar el contraste focuses more on what we do not do
• sin que cambie el contraste focuses more on what does not happen to the contrast

Both can work, but the sentence you were given highlights the idea that the contrast itself remains unchanged.

Spanish often uses the definite article with nouns where English would not.

So:

• el brillo = the brightness
• el contraste = the contrast

In contexts like image settings, controls, measurements, and general categories, Spanish commonly keeps the article:

• subir el volumen = to turn up the volume
• ajustar el brillo = to adjust the brightness
• mejorar el contraste = to improve the contrast

Leaving out the article here would sound unnatural.

Formally, cambiamos can mean either:

• we change (present)
• we changed (preterite)

So the form is ambiguous by itself.

In this sentence, many learners will naturally read it as present:

• We change the brightness without the contrast changing.

Why? Because the rest of the sentence feels like a general process or description.

If you clearly wanted a past meaning, Spanish would very often say:

• Cambiamos el brillo sin que cambiara el contraste.

So although cambiamos can technically be past too, in this sentence it is most naturally understood as present.

Yes, both are possible:

• sin que cambie el contraste
• sin que el contraste cambie

Spanish allows more flexibility in word order than English. In subordinate clauses like this one, putting the verb before the subject is very normal.

The version in your sentence sounds smooth and natural. The alternative is also correct, but may place slightly more emphasis on el contraste.

Because sin already contains the negative idea of without.

So:

• sin que cambie el contraste = without the contrast changing

Adding no would usually be wrong or at least not what you want here. In standard usage, sin que already does the job by itself.

Not quite.

In the first part:

• Cambiamos el brillo
• Here cambiar is transitive: someone changes something
• Direct object: el brillo

In the second part:

• sin que cambie el contraste
• Here cambiar is effectively intransitive
• el contraste is the thing that changes

So the same verb appears twice, but with two slightly different uses:

• change something
• something changes

That is very common in Spanish, just as in English.

Because the sentence is being understood as a present-time or general statement.

• Cambiamos el brillo sin que cambie el contraste.

If the main idea were clearly in the past, Spanish would usually shift the subordinate verb too:

• Cambiamos el brillo sin que cambiara el contraste.

So the choice of cambie matches the idea of a present/general action.