Перед сном лучше выключить телефон и проветрить комнату.

Because перед normally requires the instrumental case when it means before / in front of.

So:

• сон = sleep
• сном = instrumental singular of сон

That is why Russian says:

• перед сном = before sleep / before going to bed

This is a very common pattern:

• перед уроком = before the lesson
• перед работой = before work
• перед встречей = before the meeting

So перед сном is just the grammatically correct case form after перед.

Yes, literally it means before sleep, but in natural English it usually means:

• before going to bed
• before you fall asleep
• at bedtime

Russian often uses сон (sleep) where English would use a more idiomatic phrase like before bed.

So the phrase is literal, but its natural meaning is broader than just the moment immediately before sleeping.

This is a very common Russian structure:

• лучше + infinitive = it is better to do... / you'd better do...

So:

• лучше выключить телефон = it is better to turn off the phone
• лучше проветрить комнату = it is better to air out the room

In this sentence, there is no explicit subject like you. Russian often uses this kind of impersonal recommendation.

So the whole sentence has the feel of:

• Before bed, it’s better to turn off your phone and air out the room.

It sounds like general advice, not a direct command.

They are perfective because the sentence refers to single completed actions:

• выключить = to switch off, to turn off completely
• проветрить = to air out, to ventilate fully

The idea is not be turning off or be airing out in a general ongoing sense. It is:

• do this one action,
• complete it,
• then go to sleep.

That is why perfective is natural here.

Compare:

• лучше выключить телефон = better to turn the phone off
• лучше выключать телефон перед сном = better to turn the phone off before sleep as a regular habit

So perfective here focuses on the specific completed act.

Because Russian often leaves out the subject when it is obvious, general, or not important.

This sentence gives general advice, so Russian does not need to say:

• вам = to you
• тебе = to you
• вы / ты = you

The meaning is understood from context.

So:

• Перед сном лучше выключить телефон...

naturally means something like:

• Before bed, it’s better to turn off your phone...
• You should turn off your phone before bed...

Russian often sounds more natural than English in these subjectless recommendation structures.

They are direct objects, so they are in the accusative case.

• выключить телефон = turn off the phone
• проветрить комнату = air out the room

For телефон:

• it is masculine inanimate
• masculine inanimate nouns often look the same in nominative and accusative singular
• so телефон stays телефон

For комната:

• nominative: комната
• accusative singular: комнату

So:

• комната → комнату

This is normal first-declension noun behavior.

It applies to both:

• лучше выключить телефон и проветрить комнату

This means:

• it is better to turn off the phone and air out the room

Russian often uses one лучше with a series of infinitives joined by и (and).

If needed, you could repeat it, but it is not necessary:

• лучше выключить телефон и лучше проветрить комнату

That version is possible in some contexts, but it sounds less natural here. One лучше is enough.

Проветрить means:

• to air out
• to ventilate
• to let fresh air into

So проветрить комнату means opening a window or otherwise letting fresh air circulate in the room.

It is a very common Russian verb in everyday life.

Related words:

• ветер = wind
• проветривать = imperfective, to air out / to ventilate regularly or repeatedly
• проветрить = perfective, to air out once, successfully

So in this sentence, проветрить suggests one complete action before sleeping.

Yes. Russian word order is flexible, and this sentence could be rearranged depending on emphasis.

The original:

• Перед сном лучше выключить телефон и проветрить комнату.

is very natural because it begins with the time phrase before bed.

Other possible orders include:

• Лучше перед сном выключить телефон и проветрить комнату.
• Телефон и комнату лучше перед сном выключить и проветрить.
  This one is much less natural stylistically, but grammatically possible in a special context.

The original order is best for neutral advice:

1. set the time frame,
2. give the recommendation.

Usually it means turn the phone off or switch it off.

That is a bit stronger than:

• убрать телефон = put the phone away
• отложить телефон = put the phone aside
• не пользоваться телефоном = not use the phone

So выключить телефон normally means the device itself is switched off, not just ignored.

In real life, depending on context, some speakers might use it loosely, but the core meaning is still turn it off.

Not exactly a command. It sounds more like:

• advice
• a recommendation
• a general health tip

Because of лучше + infinitive, the tone is softer than the imperative.

Compare:

• Выключите телефон и проветрите комнату. = Turn off the phone and air out the room.
  This is a direct instruction or command.

• Перед сном лучше выключить телефон и проветрить комнату. = Before bed, it’s better to turn off the phone and air out the room.
  This is advice.

So the grammar helps create a gentler tone.