Какой бы ни был день, перед сном я всё равно выключаю телефон.

It is a concessive phrase meaning no matter what kind of day it is or whatever the day is like.

The basic idea is:

• какой = what kind of
• бы ни = part of a fixed pattern meaning no matter...
• был = the form used here with бы
• день = day

So the whole first part sets up a condition that does not affect the main action: regardless of the day, the speaker still switches off the phone before sleep.

Because какой бы ни... is a very common Russian pattern meaning no matter what kind...

You will see the same structure with other question words too:

• кто бы ни... = no matter who...
• что бы ни... = no matter what...
• где бы ни... = no matter where...
• как бы ни... = no matter how...

Here:

• бы gives a hypothetical/generalizing flavor
• ни helps create the concessive meaning regardless of...

Together, they form one idiomatic structure. You normally would not replace ни with не here.

This is a very common point of confusion.

In Russian, the past-tense form can appear with бы in clauses that are not actually past. In that case, it is not functioning as a simple past tense like was. Instead, it is part of an irrealis or generalized construction.

So какой бы ни был день does not mean whatever kind of day it was in a normal past sense. It means something more like:

• whatever the day may be like
• no matter what kind of day it is

This is idiomatic Russian grammar, not a literal past-time statement.

Because какой agrees with день in:

• masculine
• singular
• nominative

Think of какой день = what kind of day. That is the core phrase, and бы ни был is inserted into that structure.

So:

• какой день = what kind of day
• какой бы ни был день = no matter what kind of day it is

For a learner, the safest approach is to remember какой бы ни был + noun as a set pattern.

Here всё равно means anyway, all the same, or regardless.

It emphasizes that the speaker does the action no matter what. In other words:

• the day may be good
• the day may be bad
• the day may be exhausting

—but the result is the same: before sleep, the speaker switches off the phone.

Important: всё равно does not necessarily mean emotional indifference here. It does not mean the speaker doesn’t care about the day. It simply means the routine stays the same.

Because the preposition перед normally takes the instrumental case when it means before or in front of.

The noun is сон = sleep.

Its instrumental singular form is сном.

So:

• перед сном = before sleep
• natural English translation: before going to sleep or before bed

This is a very common expression in Russian.

Because the sentence describes a habitual, repeated action.

выключаю here means something like:

• I switch off
• I turn off
• I switch off as a routine

Russian often uses the imperfective present for regular habits.

Compare:

• выключаю = I do it regularly / habitually
• выключу = I will switch it off once / in the future
• выключил = I switched it off once / in the past

So the imperfective is exactly what you expect for before bed, I always switch off my phone.

Because Какой бы ни был день is a subordinate clause, and the main clause comes after it:

• Какой бы ни был день, ...
• перед сном я всё равно выключаю телефон

Russian normally separates this kind of introductory subordinate clause with a comma.

So the comma is standard punctuation here.

No, Russian word order is fairly flexible.

This version is natural because it puts the concessive idea first:

• Какой бы ни был день, ...

That gives the sentence a nice no matter what... opening.

The second part could also be rearranged a bit, for example:

• Перед сном я всё равно выключаю телефон
• Я всё равно выключаю телефон перед сном

These versions are also understandable, but the given sentence has a clear and natural emphasis: first the circumstance, then the speaker’s unchanging habit.

Yes, in modern everyday Russian, выключаю телефон will very often be understood as I turn off my phone, usually meaning a mobile phone.

If someone wanted to be extra specific, they could say:

• мобильный телефон
• смартфон

But in ordinary speech, just телефон is often enough.