Вместо того чтобы смотреть сериал до полуночи, я дочитала ещё две главы и легла пораньше.

Вместо того чтобы is a set expression meaning instead of doing something.

In this sentence, it introduces the action that did not happen:

Вместо того чтобы смотреть сериал до полуночи...
= Instead of watching a TV series until midnight...

A very common pattern is:

вместо того чтобы + infinitive

So here:

• смотреть = to watch
• the whole phrase = instead of watching

You should learn вместо того чтобы as a chunk rather than translating each word separately.

Here вместо того чтобы is treated as one whole compound conjunction, so it is not split up.

That is why the sentence is written:

Вместо того чтобы смотреть сериал до полуночи, я...

The comma appears after the whole introductory clause, before the main clause:

• subordinate clause: Вместо того чтобы смотреть сериал до полуночи
• main clause: я дочитала ещё две главы и легла пораньше

So the comma marks the boundary between those two parts.

Смотреть is imperfective, and that fits the meaning here because it refers to the activity/process of watching, not to a completed result.

The idea is:

• she could have spent time watching a series until midnight
• but instead she did something else

Imperfective is very natural after вместо того чтобы when you mean instead of doing X as an activity.

A perfective verb like посмотреть would sound more like watch something once / for a bit / to completion, which is not the main point here.

The preposition до takes the genitive case.

Base form:

• полночь = midnight

Genitive singular:

• полуночи

So:

• до полуночи = until midnight

This is a very common pattern in Russian:

• до утра = until morning
• до вечера = until evening
• до дома = as far as home / to home

Дочитала is the past tense, feminine singular form of дочитать.

It is a perfective verb, and the prefix до- often gives the idea of finishing something or bringing it to the end.

So дочитала ещё две главы means something like:

• finished reading two more chapters
• read through two additional chapters to the end

This is different from:

• читала = was reading / read (process, no completion emphasized)
• прочитала = read (completed), more neutral
• дочитала = finished reading / kept reading until finished

Here дочитала suggests a completed reading action.

Here ещё means more / another / additional, not still.

So:

ещё две главы = two more chapters

Russian ещё is a very flexible word. It can mean different things depending on context:

• ещё две главы = two more chapters
• ещё не знаю = I still don’t know
• ещё лучше = even better

In this sentence, the meaning is clearly additional.

Because глава is a feminine noun.

For the number 2, Russian uses:

• два with masculine and neuter nouns
• две with feminine nouns

So:

• два стола = two tables
• два окна = two windows
• две главы = two chapters

Also, after 2, 3, 4, the noun usually appears in the genitive singular:

• глава → главы

So the full form is:

• ещё две главы

Russian past tense verbs agree with the subject in gender and number.

Since the speaker is female, the verbs are feminine singular:

• дочитала
• легла

If the speaker were male, you would get:

• дочитал
• лёг

So the sentence itself tells you that the narrator/speaker is a woman.

Легла is the past tense of лечь, which is a perfective verb meaning to lie down / go to bed.

It is used here because the sentence presents a single completed action:

• she finished reading
• then she went to bed

That sequence strongly favors perfective verbs.

By contrast, ложилась is imperfective and would usually suggest:

• repeated action
• process
• background description

So in this sentence, легла is the natural choice.

Пораньше means a bit earlier or earlier than usual.

It is often used in everyday Russian when talking about doing something slightly earlier in a practical sense:

• лечь пораньше = go to bed a little earlier / go to bed early

Compared with раньше:

• раньше often means simply earlier and may sound more comparative
• пораньше often sounds softer and more idiomatic, especially for daily habits

So:

• легла раньше = went to bed earlier
• легла пораньше = went to bed a bit earlier / earlier than usual

Yes, it could be omitted.

Russian often leaves out subject pronouns when the meaning is already clear from the verb form:

• дочитала already shows I and also shows feminine gender in context

So you could say:

Вместо того чтобы смотреть сериал до полуночи, дочитала ещё две главы и легла пораньше.

That said, я is perfectly natural here. It can add a little clarity or emphasis, especially after the opening contrast:

• Instead of doing that, I did this.

So the pronoun is optional, but not strange.

Because the sentence tells a sequence of completed actions.

She:

1. finished reading two more chapters
2. went to bed earlier

Perfective verbs are very common when Russian describes one completed event after another.

So the aspect pattern in the sentence is very natural:

• смотреть — imperfective, because it names the alternative activity
• дочитала, легла — perfective, because they describe what actually happened as completed actions