Мне нравится расчёсывать волосы перед сном.

Because нравиться works differently from English to like.

Russian structures it more like This is pleasing to me:

• мне = to me
• нравится = is pleasing / is liked

So:

• Мне нравится расчёсывать волосы. = I like brushing my hair. = literally, Brushing hair is pleasing to me.

That is why the person experiencing the feeling appears in the dative case: мне.

Because мне is the dative case of я.

With нравиться, the thing or action being liked is the grammatical subject, and the person who likes it is in the dative:

• мне нравится музыка = I like music
• ему нравится читать = he likes reading
• нам нравится гулять = we like walking

So in your sentence:

• мне = to me
• нравится расчёсывать волосы = brushing hair is pleasing

Because here the thing being liked is not a plural noun like волосы, but the infinitive phrase расчёсывать волосы.

An infinitive in Russian behaves as a singular idea, so Russian uses нравится:

• Мне нравится читать. = I like reading.
• Мне нравится готовить. = I like cooking.
• Мне нравится расчёсывать волосы. = I like brushing my hair.

Compare that with a plural noun:

• Мне нравятся книги. = I like books.

So нравится is singular because the subject is the action расчёсывать волосы, not the plural noun волосы by itself.

After мне нравится, Russian often uses an infinitive to express liking an activity:

• Мне нравится читать = I like to read / I like reading
• Мне нравится плавать = I like swimming
• Мне нравится расчёсывать волосы = I like brushing my hair

This is very normal Russian.

So the pattern is:

мне нравится + infinitive

It corresponds to English I like doing... or I like to do...

Because the sentence describes a habitual / general activity, not one completed action.

Russian aspect matters here:

• расчёсывать = imperfective, focusing on the process or repeated activity
• расчесать = perfective, focusing on completing the action once

With нравится + infinitive, Russian usually uses the imperfective when talking about something you generally enjoy doing:

• Мне нравится читать перед сном. = I like reading before bed.
• Мне нравится расчёсывать волосы перед сном. = I like brushing my hair before bed.

If you used the perfective, it would sound much less natural here, because the meaning is not I like to finish brushing my hair once, but I enjoy the activity / habit.

In Russian, волосы is normally used in the plural when talking about the hair on your head.

So:

• волосы = hair
• literally it is more like hairs, but in normal English we translate it as hair

Examples:

• У неё длинные волосы. = She has long hair.
• Я мою волосы. = I wash my hair.
• Я расчёсываю волосы. = I brush my hair.

Russian does have the singular волос, but that usually means a single hair.

Because in Russian, possessive words like мой / моя / мои are often omitted when it is already obvious whose body part or personal item is meant.

So:

• Мне нравится расчёсывать волосы naturally means I like brushing my hair
• not someone else’s hair, unless context suggests otherwise

Russian commonly omits the possessive with body parts when the owner is clear from context:

• Я мыл руки. = I washed my hands.
• Он закрыл глаза. = He closed his eyes.
• Она расчёсывает волосы. = She is brushing her hair.

You can say мои волосы, but it often sounds more emphatic or contrastive:

• Мне нравится расчёсывать мои волосы, а не парик. = I like brushing my own hair, not the wig.

Yes, you could, but in many contexts it is unnecessary.

• Мне нравится расчёсывать волосы is the most natural general sentence
• Мне нравится расчёсывать себе волосы adds the idea to myself / my own

The word себе can be used when Russian wants to make the beneficiary or owner more explicit, but with body parts it is often omitted if the meaning is already clear.

So:

• Мне нравится расчёсывать волосы перед сном = the natural default
• Мне нравится расчёсывать себе волосы перед сном = also possible, but slightly more explicit

They are related, but not identical.

• расчёсывать волосы = to brush/comb the hair
• причёсываться / причёсывать волосы = to comb/style/tidy one’s hair

Very roughly:

• расчёсывать focuses more on the act of running a comb or brush through the hair
• причёсывать often suggests arranging it neatly

So your sentence with расчёсывать clearly refers to the brushing/combing action itself.

Сном is the instrumental case of сон (sleep).

The preposition перед normally takes the instrumental:

• перед домом = in front of the house
• перед ужином = before dinner
• перед сном = before sleep / before bed

In natural English, перед сном is usually translated as before bed or before going to sleep.

So:

• сон = sleep
• сном = instrumental form after перед

Not quite.

• перед сном is the normal idiomatic way to say before bed / before sleeping
• до сна is grammatical, but much less natural in this everyday context

A learner should usually remember:

• перед сном = the standard expression for before bed

Similarly:

• после сна = after sleep
• во сне = in a dream / while asleep

Yes. Russian word order is flexible, though some orders sound more neutral than others.

The most neutral version is:

• Мне нравится расчёсывать волосы перед сном.

But you could also hear:

• Перед сном мне нравится расчёсывать волосы.
  • emphasizes before bed
• Волосы мне нравится расчёсывать перед сном.
  • stronger focus on hair

Russian changes word order mainly for emphasis, topic, and style, not usually because the grammar changes.

So the original sentence is a very natural, neutral choice.

In normal speech, нравится is often pronounced roughly like NRA-vit-sa.

A few useful points:

• the stress is on нра́
• -ться / -тся in fast speech often sounds close to ца
• so нравится may sound approximately like нра́вица

This is normal spoken Russian. The spelling stays нравится, but the pronunciation is often reduced.

Likewise:

• учиться often sounds like учица
• боится often sounds like боица

That is a pronunciation feature, not a different grammar form.