Сегодня я пойду спать пораньше, чтобы завтра не опоздать на работу.

Because пойду спать means I’ll go to bed / I’ll go to sleep, not simply I’ll be sleeping.

• пойду is the future of пойти (to go)
• спать is to sleep

Together, пойти спать is a very common Russian expression meaning to go to bed or to go sleep.

Compare:

• Я пойду спать. = I’ll go to bed.
• Я буду спать. = I’ll be sleeping.

So in this sentence, the speaker means they will go to bed earlier, not just describe a future state of sleeping.

Пораньше means earlier or a bit earlier.

It comes from рано (early). The form раньше means earlier, and adding по- often makes it sound a bit softer or less absolute:

• раньше = earlier
• пораньше = a little earlier / somewhat earlier / nice and early

So:

• пойду спать пораньше = I’ll go to bed earlier

It often sounds natural in everyday speech when someone means earlier than usual.

Чтобы introduces a purpose clause. It means so that, in order to, or so as not to.

In this sentence:

• Сегодня я пойду спать пораньше, чтобы завтра не опоздать на работу.
• Today I’ll go to bed earlier so that I won’t be late for work tomorrow.

So the first part gives the action, and the чтобы clause gives the goal/purpose of that action.

After чтобы, Russian can use either:

1. a past-tense-like finite form
2. an infinitive

The infinitive is especially common when the subject is the same as in the main clause, or when the action is expressed more generally.

Here the subject is still я, so:

• чтобы завтра не опоздать на работу = so as not to be late for work tomorrow

You could also say:

• чтобы я завтра не опоздал на работу

That is also correct, but the infinitive version is often more compact and natural in this kind of sentence.

Because опоздать is perfective and refers to one complete event: being late on a particular occasion.

Here the speaker means:

• so that tomorrow I won’t be late

That is a single future event, so perfective опоздать fits well.

Compare:

• опоздать = to be late once / to arrive late
• опаздывать = to be habitually late / to keep being late

So:

• не опоздать завтра = not be late tomorrow
• не опаздывать would sound more like not to be late regularly

They are an aspect pair:

• опоздать = perfective
• опаздывать = imperfective

Use опоздать for a single completed event:

• Я опоздал на работу. = I was late for work.

Use опаздывать for repeated, habitual, or ongoing lateness:

• Я часто опаздываю на работу. = I’m often late for work.

In your sentence, the speaker is worried about one specific morning tomorrow, so опоздать is the right choice.

Работу is accusative singular of работа.

It appears after на because Russian uses:

• на + accusative for motion toward a place
• на + prepositional for location at/on a place

So:

• на работу = to work
• на работе = at work

This is an idiomatic expression in Russian:

• идти на работу = to go to work
• опоздать на работу = to be late for work

Even though English says for work, Russian uses на работу.

Because they refer to different parts of the situation:

• Сегодня = when the speaker will go to bed earlier
• завтра = when the speaker wants not to be late for work

So the timeline is:

• Today: go to bed early
• Tomorrow: avoid being late for work

Russian often places time words near the part they relate to most clearly.

No, Russian word order is fairly flexible.

The sentence:

• Сегодня я пойду спать пораньше, чтобы завтра не опоздать на работу.

is natural and neutral.

But other orders are also possible, for example:

• Я сегодня пойду спать пораньше, чтобы завтра не опоздать на работу.
• Чтобы завтра не опоздать на работу, сегодня я пойду спать пораньше.

The main meaning stays the same, but the focus changes slightly.

• Starting with Сегодня emphasizes today
• Starting with Чтобы завтра не опоздать... emphasizes the purpose

Yes, very often.

Russian frequently drops subject pronouns when the verb already makes the subject clear.

So you could say:

• Сегодня пойду спать пораньше, чтобы завтра не опоздать на работу.

This still clearly means I’ll go to bed earlier today...

Including я is not wrong at all. It can add a bit of clarity, contrast, or emphasis.

You can absolutely say лечь спать, and in many contexts it may even sound more directly like go to bed / lie down to sleep.

Compare:

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

So this would also be natural:

• Сегодня я лягу спать пораньше, чтобы завтра не опоздать на работу.

Both are common.
Very roughly:

• пойти спать focuses on the decision/movement into that activity
• лечь спать focuses a bit more on actually lying down and going to bed

Because in Russian, не normally goes directly before the word it negates.

Here it negates the infinitive:

• не опоздать = not to be late

So:

• чтобы завтра не опоздать на работу = so as not to be late for work tomorrow

That is the normal placement.

Because сегодня just means today; it does not mean the action has already happened.

The speaker is talking about a plan later today:

• Сегодня я пойду спать пораньше = Today I’ll go to bed earlier

So сегодня gives the time frame, and пойду shows that the action is still in the future relative to the moment of speaking.

Not exactly. It means I’ll go to bed earlier or I’ll go sleep earlier.

Russian is not directly saying when the person will actually fall asleep. It is talking about the action of going to bed.

If you wanted to talk specifically about falling asleep, you would use a different verb, for example:

• заснуть = to fall asleep

So:

• пойду спать пораньше = go to bed earlier
• засну пораньше = fall asleep earlier

Those are related, but not the same.

Very often, yes, but not always.

In this sentence it clearly expresses purpose:

• I’ll go to bed earlier so that I won’t be late tomorrow.

But чтобы can also appear in other constructions, for example in requests or clauses with a desired result.

Still, for a learner, it is very useful to remember that in sentences like this one, чтобы usually means:

• so that
• in order to

That is exactly how it works here.