Nos cepillamos el cabello antes de dormir.

Because cepillarse is a reflexive verb here. In Spanish, actions people do to their own body or personal grooming often use a reflexive pronoun:

• me cepillo = I brush my hair / I brush myself
• te cepillas = you brush your hair
• nos cepillamos = we brush our hair

So nos matches we. Literally, Nos cepillamos el cabello is something like We brush ourselves the hair, but in natural English it means We brush our hair.

In Spanish, when the owner is already clear from a reflexive pronoun, Spanish usually uses the definite article (el, la, los, las) instead of a possessive like my, your, our.

So Spanish prefers:

• Me lavo las manos = I wash my hands
• Nos cepillamos el cabello = We brush our hair

Using nuestro cabello is possible, but it sounds less natural in an everyday sentence unless you want extra emphasis or contrast.

Because cabello often works like a mass noun here, meaning hair in general rather than individual hairs.

So:

• el cabello = hair, one’s hair as a whole
• los cabellos = individual hairs, strands of hair

In normal speech, when talking about grooming, el cabello is the natural choice.

Both can mean hair, but there is a small difference in tone:

• cabello often sounds a little more careful, formal, or specifically like the hair on your head
• pelo is very common and often more everyday

In many Latin American contexts, both are understood perfectly. A very common version of this sentence is also:

• Nos cepillamos el pelo antes de dormir.

That sounds very natural too.

Yes. Cepillamos can be either:

• present: we brush
• preterite: we brushed

The form is spelled the same in both tenses.

In this sentence, the meaning is usually understood as present / habitual because of the context: antes de dormir suggests a routine, like something we do before going to bed.

If the context were a story about yesterday, it could mean we brushed.

After antes de, Spanish normally uses an infinitive when the subject is the same.

So:

• antes de dormir = before sleeping / before going to sleep

Not:

• antes de dormimos ❌

This is similar to English using before sleeping rather than before we sleep in some contexts.

If Spanish wants to introduce a different subject, it often uses:

• antes de que + subjunctive

For example:

• Antes de que los niños duerman... = Before the children sleep...

Grammatically, cepillar exists without the reflexive pronoun, but in this context it would sound unusual.

• Nos cepillamos el cabello = we brush our own hair
• Cepillamos el cabello by itself sounds incomplete or odd, as if you are just saying we brush hair

For personal grooming, Spanish normally uses the reflexive form.

Non-reflexive cepillar is more natural when the object is something external:

• Cepillamos al perro = We brush the dog
• Cepillo la chaqueta = I brush the jacket

Here it normally means ourselves—more specifically, each of us brushes our own hair.

Reflexive pronouns in plural forms can sometimes also allow a reciprocal idea (each other), but in this sentence the normal interpretation is the grooming routine meaning: we brush our own hair.

If someone really wanted to emphasize each other’s hair, they would usually make that clearer in some way.

Because Spanish often drops subject pronouns when they are already clear from the verb ending.

• Nos cepillamos already tells you the subject is we
• adding nosotros is possible, but usually unnecessary

So:

• Nos cepillamos el cabello... = normal
• Nosotros nos cepillamos el cabello... = possible, but more emphatic

Yes. Spanish word order is somewhat flexible.

You can also say:

• Antes de dormir, nos cepillamos el cabello.

That means the same thing. This version just puts the time expression first for emphasis or flow.

Both orders are natural.