Мне пора бы лечь спать, но я всё ещё читаю книгу.

Because пора is used in an impersonal construction.

• Мне пора literally works like To me, it is time
• Natural English translation: It’s time for me / I should be going to bed

So мне is in the dative case, not the nominative. Russian often uses dative with expressions that describe a person’s state, need, age, or obligation:

• мне холодно = I’m cold
• мне нужно идти = I need to go
• мне пора = it’s time for me

You do not say я пора.

Пора means it is time or time to.

In this sentence, Мне пора бы лечь спать means something like:

• It’s about time I went to bed
• I really should go to bed

A few useful patterns:

• Пора идти = It’s time to go
• Мне пора домой = It’s time for me to go home
• Нам пора спать = It’s time for us to sleep

So пора is a very common way to talk about the right time to do something.

Бы adds a softer, more tentative, or slightly reflective meaning.

Compare:

• Мне пора лечь спать = It’s time for me to go to bed
• Мне пора бы лечь спать = I really should go to bed / It’s about time I went to bed

With бы, the speaker sounds like they know what they ought to do, but they are not doing it yet. That fits perfectly with the second half of the sentence:

• но я всё ещё читаю книгу = but I’m still reading a book

So бы helps create the feeling of: I should be in bed by now... but I’m still reading.

Because лечь is the perfective verb, and here it refers to a single completed action: going to bed.

The pair is:

• ложиться = imperfective
• лечь = perfective

In this sentence, the speaker means: I should go to bed now
—that is, perform one concrete action.

So лечь спать is very natural.

Compare:

• Я обычно ложусь спать в 11. = I usually go to bed at 11.
  (habit, repeated action)
• Мне пора лечь спать. = It’s time for me to go to bed.
  (one specific action now)

In Russian, лечь спать is a very common expression meaning to go to bed to sleep.

Literally:

• лечь = to lie down / go lie down
• спать = to sleep

Together they mean:

• go to bed
• lie down to sleep

Russian often uses an infinitive of purpose like this. Similar examples:

• сесть поесть = sit down to eat
• пойти гулять = go out for a walk
• остаться работать = stay to work

So лечь спать is completely idiomatic.

Here, всё ещё means still.

• Я всё ещё читаю книгу = I’m still reading the book / a book

On its own, ещё can mean several things depending on context:

• more / another
• again
• still

But всё ещё is a very clear way to say still and often emphasizes that the situation continues longer than expected.

Compare:

• Я ещё читаю = I’m still reading / I’m reading some more
  (can depend on context)
• Я всё ещё читаю = I’m still reading
  (very clear continuation)

Because the action is ongoing.

Читать is imperfective, and читаю means:

• I am reading
• I read (depending on context)

In this sentence, because of всё ещё, the meaning is clearly: I’m still reading.

The speaker is describing an activity in progress, not a finished result. That is exactly what the imperfective aspect is used for.

If you used a perfective verb here, it would not fit the meaning of an action that is still continuing.

Because книга is the direct object of читаю.

The verb читать takes a direct object:

• читать книгу = to read a book

Since книга is a feminine noun ending in -а, its accusative singular changes to -у:

• nominative: книга
• accusative: книгу

So:

• Я читаю книгу = I am reading a book / the book

This is a standard accusative pattern for feminine inanimate nouns.

It can mean either, depending on context.

Russian does not have articles like a and the, so книгу by itself could be:

• a book
• the book

In this sentence, English could naturally translate it as either:

• I’m still reading a book
• I’m still reading the book

The broader context would tell you which one is more natural.

Yes, Russian often omits subject pronouns when they are clear from the verb ending. So you could say:

• ...но всё ещё читаю книгу

and it would still mean: ...but I’m still reading a book

However, я is often included for clarity, contrast, or emphasis. Here it helps create a nice contrast:

• Мне пора бы лечь спать, но я всё ещё читаю книгу.

The speaker is almost stressing: but I am still reading.

So the pronoun is not required, but it sounds natural.

Because но means but, and it joins two clauses.

The two parts are:

• Мне пора бы лечь спать
• я всё ещё читаю книгу

Since these are two separate clauses, Russian puts a comma before но:

• ..., но ...

This is very similar to English punctuation with but.

Yes. Russian word order is flexible, though different orders can change the emphasis.

The given sentence is natural:

• Мне пора бы лечь спать, но я всё ещё читаю книгу.

Other possible versions:

• Я всё ещё читаю книгу, но мне пора бы лечь спать.
• Мне пора бы спать, но я всё ещё читаю книгу.

The basic meaning stays the same, but the focus shifts a little.

In the original sentence, the speaker starts with what they should do, then contrasts it with what they are actually doing.

Yes, it is natural, especially in conversational or reflective speech.

It sounds like someone thinking or saying to themselves:

• I really should go to bed by now

It has a slightly self-aware tone because of бы. Some close alternatives are:

• Мне пора лечь спать. = It’s time for me to go to bed.
• Мне бы пора спать. = I really ought to be sleeping by now.
• Мне пора спать. = It’s time for me to sleep / go to bed.

So the original sentence is natural and expressive, especially when followed by но я всё ещё читаю книгу.