Чтобы успеть на ранний поезд, мы выехали затемно.

Here чтобы introduces a purpose clause. It means in order to or so that.

So:

• Чтобы успеть на ранний поезд = In order to catch / make the early train
• мы выехали затемно = we left while it was still dark

This is a very common pattern in Russian:

• чтобы + infinitive when the subject is the same
• чтобы + past-tense verb when the clause has its own subject or is stated more explicitly

Because the subject is the same in both parts of the sentence: мы are the ones who left, and мы are also the ones trying to catch the train.

In Russian, when the same person does both actions, чтобы + infinitive is very natural:

• Чтобы успеть..., мы выехали...

You can also say:

• Чтобы мы успели на ранний поезд, мы выехали затемно.

That is grammatical, but it sounds more explicit and a bit heavier. In ordinary speech, Russian often prefers the shorter infinitive construction when the subject is clear.

Успеть is perfective because it refers to a completed result: successfully being in time for something.

The contrast is roughly:

• успевать = to be managing / to have enough time in general or repeatedly
• успеть = to manage to do something on a particular occasion

In this sentence, they left early in order to achieve one concrete result: to make the early train. That is why успеть is the natural choice.

На ранний поезд is accusative case after the preposition на.

With успеть, Russian often uses на for events or transport departures:

• успеть на поезд = to catch / make the train
• успеть на автобус = to catch the bus
• успеть на самолёт = to make the plane
• успеть на встречу = to make it to the meeting

So на поезд is the normal idiomatic pattern here.

Also, поезд is masculine and inanimate, so in the accusative singular it looks the same as the nominative:

• nominative: ранний поезд
• accusative: ранний поезд

That is why the form does not visibly change.

Выехали is the past plural of выехать, a perfective verb meaning to set out, to leave, or to depart, usually with the idea of going by some means of transport or leaving from a place.

In this sentence, it suggests something like:

• we set off before dawn
• we left while it was still dark

Compared with some related verbs:

• поехали = started going / set off
• уехали = left, went away
• выехали = set out, drove out, left from a place

Here выехали fits well because the focus is on departing early from where they were.

Because the subject is мы = we.

Russian past tense agrees with number, and in the singular it also agrees with gender:

• я выехал / выехала
• ты выехал / выехала
• мы выехали
• они выехали

So -ли marks plural past.

Затемно is an adverb. It means before dawn, while it was still dark, or in the dark / before daylight.

Important points:

• It does not change form.
• It describes when they left.
• It is a bit more literary or expressive than a completely neutral phrase like до рассвета.

So:

• мы выехали затемно = we left before it got light

It is understandable and normal Russian, but it can sound a little more vivid or slightly literary than the most neutral everyday wording.

Possible alternatives:

• до рассвета = before dawn
• очень рано утром = very early in the morning
• когда было ещё темно = when it was still dark

So затемно is not strange, but it has a compact, slightly expressive feel.

Because Чтобы успеть на ранний поезд is a subordinate clause of purpose, and Russian separates such clauses with a comma.

So the structure is:

• Чтобы успеть на ранний поезд, = subordinate purpose clause
• мы выехали затемно. = main clause

If you reverse the order, you still keep the comma:

• Мы выехали затемно, чтобы успеть на ранний поезд.

Yes. Russian word order is flexible.

This sentence can also be written as:

• Мы выехали затемно, чтобы успеть на ранний поезд.

Both versions are natural. The difference is mostly one of emphasis:

• Чтобы успеть на ранний поезд, мы выехали затемно.
  Focuses first on the purpose.
• Мы выехали затемно, чтобы успеть на ранний поезд.
  Starts with the main action and adds the purpose afterward.

Often, yes. Выехать usually suggests departure involving movement from a place, frequently by vehicle.

For example, it can sound natural if they:

• drove out
• set off in a car
• left on some transport

But in many contexts, English does not need that detail, so translating it simply as left or set out is fine.

If the sentence used вышли, that would more strongly suggest went out / left on foot.

Yes. That is a good close translation.

Depending on context, natural English versions include:

• to be in time for the early train
• to make the early train
• to catch the early train

All of these match the Russian well. The Russian wording focuses on the idea of managing to be in time, which English often turns into catch or make in this context.