Когда духовка снова заработала, мама допекла пирог до конца.

Because Когда духовка снова заработала is a subordinate time clause: When the oven started working again. In Russian, subordinate clauses are normally separated from the main clause by a comma.

So the sentence is structured like this:

• Когда духовка снова заработала = subordinate clause
• мама допекла пирог до конца = main clause

Russian does this very regularly, even when English punctuation can feel less strict.

Когда means when and introduces a time clause.

In this sentence, it tells us at what point the main action happened:

• Когда духовка снова заработала = When the oven started working again
• мама допекла пирог до конца = Mom finished baking the pie

So когда connects the two events in time.

Here заработала means started working / began functioning again.

This is a very common use of заработать with machines, devices, systems, etc.:

• Компьютер заработал. = The computer started working.
• Лифт снова заработал. = The elevator started working again.

This is probably confusing because заработать can also mean to earn:

• заработать деньги = to earn money

But in this sentence, with духовка as the subject, it clearly means to begin functioning.

Because работала and заработала mean different things.

• работала = was working / worked
• заработала = started working

Here the point is that the oven had not been working, and then it began to work again. So заработала is exactly the right verb.

Compare:

• Когда духовка работала... = When the oven was working...
  This just describes its state.
• Когда духовка снова заработала... = When the oven started working again...
  This marks a change of state.

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

The subject here is духовка, which is a feminine noun. So the past tense is feminine:

• masculine: заработал
• feminine: заработала
• neuter: заработало
• plural: заработали

The same thing happens in the main clause:

• мама допекла — мама is feminine, so the verb is feminine too.

Снова means again.

So:

• духовка снова заработала = the oven started working again

Its position is fairly natural here, before the verb. Russian word order is flexible, but this placement sounds neutral and common.

You could also see:

• Когда снова заработала духовка...
• Когда духовка заработала снова...

Those are possible, but духовка снова заработала is the most straightforward neutral order.

Допекла means finished baking or baked the rest of the way.

It comes from допечь, which uses the prefix до-. That prefix often gives the idea of bringing something to completion, doing the rest, or continuing until the end.

So:

• пекла = was baking / baked
• допекла = finished baking

This suggests the pie was already partly baked, but the process had been interrupted — presumably because the oven stopped working. Once the oven worked again, Mom completed the baking.

It is in the accusative. The reason it looks unchanged is that пирог is an inanimate masculine noun, and for that group the accusative singular usually looks the same as the nominative singular.

So:

• nominative: пирог
• accusative: пирог

This is normal in Russian.

Compare with an animate masculine noun:

• nominative: кот
• accusative: кота

So in мама допекла пирог, пирог is absolutely the direct object.

Literally, до конца means to the end.

So:

• допекла пирог до конца = finished baking the pie all the way through / to the end

You are right that допекла already contains the idea of completion. Adding до конца gives extra emphasis and makes the completion especially clear.

It is a bit like saying in English:

• She finished baking the pie
• She finished baking the pie all the way to the end

The second is more explicit and emphatic.

Because both actions are viewed as completed events.

• заработала = the oven started working at a specific moment
• допекла = Mom finished baking the pie

Russian often uses the perfective aspect when talking about one completed event happening and then another completed event following it.

This creates a clear sequence:

1. the oven started working again
2. Mom finished baking the pie

If the imperfective were used, the sentence would describe an ongoing process or repeated action instead of these specific completed events.

Yes. Russian word order is flexible, although some versions sound more neutral than others.

The original sentence is very natural:

• Когда духовка снова заработала, мама допекла пирог до конца.

You could also say:

• Мама допекла пирог до конца, когда духовка снова заработала.

That still means basically the same thing, but now the main clause comes first.

Russian speakers often move words around for emphasis, rhythm, or information structure. The original version is a very standard neutral way to present the events in time order.

Yes, very strongly.

The combination of снова + заработала suggests this sequence:

• the oven had been working before
• then it stopped working
• then it started working again

And допекла suggests the pie had already been partly baked but not finished.

So even though the sentence does not explicitly say the oven broke or the baking was interrupted, that is the natural implication.

Russian often uses the noun directly instead of a pronoun when it is clear and natural to do so.

So:

• мама допекла пирог = Mom finished baking the pie

There is no need to add она. In fact, adding it would usually sound unnecessary unless you wanted contrast or emphasis.

Russian generally uses pronouns less often than English does when the subject is already clear.

Sometimes yes, but снова is the better choice here.

Both can mean again, but they are not always identical in tone:

• снова is neutral and works very well for again
• опять can also mean again, but it often carries a feeling like again, once more, yet again, and sometimes a slight emotional coloring such as annoyance

So:

• духовка снова заработала sounds neutral: the oven started working again
• духовка опять заработала is possible, but it may sound a little different in tone depending on context

For this sentence, снова is the most straightforward choice.