Sara pensava che la pizza sarebbe lievitata meglio se l’avessimo lasciata vicino alla finestra.

Pensava is the imperfetto. Here it gives background information: it describes what Sara was thinking or believed at that time.

Using ha pensato would sound more like a completed, single event: she thought in the sense of it occurred to her or she had the thought.

So in this sentence, pensava fits better because it presents Sara’s belief as an ongoing mental state in the past.

After verbs like pensare, credere, dire, and sapere, Italian often uses che to introduce a subordinate clause.

So:

• Sara pensava che... = Sara thought that...

Just like English can say that, Italian normally uses che here.

Sarebbe lievitata is the condizionale passato (past conditional).

It is formed with:

• the conditional of essere: sarebbe
• the past participle: lievitata

This tense is often used to express what would have happened under certain conditions.

Here it means the pizza would have risen better.

Because it refers to la pizza, which is feminine singular.

With verbs that use essere in compound tenses, the past participle agrees with the subject:

• la pizza sarebbe lievitata
• il pane sarebbe lievitato
• le pizze sarebbero lievitate

So lievitata matches pizza.

In this sentence, lievitare is used as an intransitive verb: the pizza rises by itself.

Many intransitive verbs of change of state or movement use essere in compound tenses, so:

• la pizza è lievitata
• la pizza sarebbe lievitata

That is why you see sarebbe lievitata, not avrebbe lievitato.

L’avessimo lasciata is the congiuntivo trapassato (pluperfect subjunctive).

It is formed with:

• imperfect subjunctive of avere: avessimo
• past participle: lasciata

This structure appears because the sentence is describing an unreal past condition:

• if we had left it near the window...

So the overall pattern is:

• would have risen = sarebbe lievitata
• if we had left it = se l’avessimo lasciata

This is a common way to express a past hypothetical idea.

That is a very good question, because learners often see different patterns.

In standard modern Italian, an unreal past condition is very commonly expressed as:

• se + congiuntivo trapassato
• main clause with condizionale passato

So this sentence follows a normal literary and standard pattern:

• se l’avessimo lasciata...
• sarebbe lievitata...

You may also hear or read other patterns in less formal speech, but this version is absolutely correct and very standard.

L’ stands for la, referring to la pizza.

So:

• l’avessimo lasciata = avessimo lasciato la pizza

Italian often replaces a direct object noun with a direct object pronoun:

• la pizza → la → l’ before a vowel

That is why la becomes l’ before avessimo.

Because the direct object pronoun la / l’ comes before the verb, and with compound tenses this often makes the past participle agree with the direct object.

Here the object is la pizza, feminine singular, so:

• l’avessimo lasciata

Compare:

• il pane → lo avessimo lasciato
• le pizze → le avessimo lasciate

So lasciata agrees with la pizza.

Vicino usually takes the preposition a:

• vicino a qualcosa = near something

Since la finestra is feminine singular, a + la becomes alla:

• vicino alla finestra

So the structure is:

• vicino a
• a + la = alla

In Italian, adverbs like meglio often come after the verb or verbal phrase.

So:

• sarebbe lievitata meglio

means:

• would have risen better

That word order is very natural in Italian. English can also do something similar: would have risen better.

Not with exactly the same nuance.

Lievitare specifically refers to dough rising because of yeast or fermentation. It is the most precise verb for bread or pizza dough.

Crescere means to grow and is much more general. In some contexts people may understand it, but lievitare is the correct and natural choice for pizza dough.

Italian is a pro-drop language, which means subject pronouns are often omitted because the verb ending already shows the subject.

Avessimo clearly means we had in this form, so Italian does not need noi.

You could say se noi l’avessimo lasciata, but it is usually unnecessary unless you want emphasis or contrast.

It is both.

1. Reported thought:
   Sara pensava che... tells you what Sara believed.

2. Hypothetical past situation:
   la pizza sarebbe lievitata meglio se l’avessimo lasciata vicino alla finestra expresses something unreal in the past — the pizza did not get left there, but Sara thought that if it had been, it would have risen better.

So the sentence combines:

• a past mental attitude
• a past unreal condition and result

Then Italian would normally use a different tense pattern.

For a present or future possibility, you might get something like:

• Sara pensa che la pizza lieviterà meglio se la lasciamo vicino alla finestra.

That means something like:

• Sara thinks the pizza will rise better if we leave it near the window.

Your original sentence is different because it looks back at a hypothetical past situation:

• would have risen
• if we had left it

So the original is specifically about something imagined after the fact.