Ona powiedziała, że powinna była wcześniej włączyć piekarnik.

Yes, you often can leave it out.

Polish usually drops subject pronouns when the verb already makes the subject clear. So:

• Ona powiedziała, że powinna była wcześniej włączyć piekarnik.
• Powiedziała, że powinna była wcześniej włączyć piekarnik.

Both can mean the same thing.

Using Ona adds one of these effects:

• emphasis
• contrast, as in She said it
• clarity, if several people are being discussed

So the pronoun is not wrong at all, but it is often optional.

Because the subject is feminine.

In the past tense, Polish verbs agree with the subject’s gender in the singular:

• on powiedział = he said
• ona powiedziała = she said

So since the sentence is about she, the correct form is powiedziała.

This is the standard way to express should have done for a feminine subject.

Breakdown:

• powinna = should / ought to
• była = part of the past construction
• włączyć = to turn on

Together:

• powinna była włączyć = she should have turned on

This structure is very common in Polish:

• powinien był zrobić = he should have done
• powinna była zrobić = she should have done
• powinni byli zrobić = they should have done

So była here is not the main meaning was. It helps build the past meaning of obligation/regret.

They do different jobs.

• powinna expresses duty, expectation, or what would have been the right thing
• była places that duty in the past: should have

Compare:

• Powinna włączyć piekarnik. = She should turn on the oven / ought to turn on the oven.
• Powinna była włączyć piekarnik. = She should have turned on the oven.

So była is what gives the sentence that clear past, hindsight meaning.

Sometimes in casual speech, yes, but it changes how explicit the meaning is.

You may hear:

• Powinna wcześniej włączyć piekarnik.

Depending on context, that can mean either:

• She should turn on the oven earlier or
• She should have turned on the oven earlier

If you want to clearly say should have, then powinna była is the safest and most standard choice.

Because after powinna była, Polish uses the infinitive.

So the pattern is:

• powinna była + infinitive
• powinien był + infinitive
• powinni byli + infinitive

Examples:

• Powinna była zadzwonić. = She should have called.
• Powinien był wyjść. = He should have left.

So włączyć stays in the infinitive because the modal idea is already carried by powinna była.

Because włączyć is perfective, and here Polish wants a completed one-time action.

Turning on the oven is treated as a single completed act, so perfective is natural:

• włączyć = to turn on, successfully / as a completed act

The imperfective włączać would suggest repetition, habit, or process, and sounds much less natural here.

Compare:

• Powinna była włączyć piekarnik. = She should have turned on the oven.
• Zawsze włączała piekarnik za późno. = She always turned the oven on too late.

So in this sentence, włączyć is the right choice.

It is in the accusative — it just happens to look the same as the nominative.

Piekarnik is a masculine inanimate noun. In the singular, masculine inanimate nouns usually have:

• nominative = accusative

So:

• piekarnik = nominative
• widzę piekarnik = accusative
• włączyć piekarnik = accusative

The form does not visibly change, but the grammatical case is still accusative.

Wcześniej means earlier.

It tells us that the action should have happened sooner.

Its position is fairly flexible. These are all natural:

• powinna była wcześniej włączyć piekarnik
• powinna była włączyć piekarnik wcześniej

The first version often sounds a bit more neutral.
The second puts slightly more focus on earlier.

So the placement can shift depending on rhythm or emphasis.

Because Polish normally puts a comma before a subordinate clause introduced by że.

So:

• Powiedziała, że... = She said that...

This is standard punctuation in Polish. English sometimes omits the comma before that, but Polish keeps it here.

Że means that and introduces the content of what she said.

So:

• Ona powiedziała = She said
• że powinna była wcześniej włączyć piekarnik = that she should have turned on the oven earlier

It connects the reporting clause to the reported statement.

Standard Polish uses powinna była, not była powinna.

So the normal order is:

• powinien/powinna/powinno + był/była/było + infinitive

Examples:

• powinien był przyjść
• powinna była zadzwonić
• powinno było działać

Using była powinna would sound wrong or very unnatural to most speakers.

You would change the gender-marked forms:

• On powiedział, że powinien był wcześniej włączyć piekarnik.

Compare:

• Ona powiedziała, że powinna była... = feminine
• On powiedział, że powinien był... = masculine

So both the reporting verb and the powinien/powinna był/była part change for gender.

Włączyć piekarnik is perfectly normal and means to turn the oven on.

Depending on context, Polish might also use more specific verbs, for example:

• nagrzać piekarnik = to heat/preheat the oven
• rozgrzać piekarnik = to warm up/preheat the oven

So if the idea is specifically preheat the oven, Polish often chooses one of those. But if the sentence just means turn the oven on, then włączyć piekarnik is absolutely fine.