Marta ha detto che avrebbe impastato il pane appena tornava dal mercato.

Here avrebbe impastato is being used for future in the past in reported speech. After a past verb like ha detto, Italian normally uses the condizionale passato to report something that was still in the future at that past moment.

So:

• Marta ha detto: Impasterò il pane
• becomes
• Marta ha detto che avrebbe impastato il pane

In natural English, that is usually just Marta said she would knead/make the bread, not necessarily would have kneaded.

Because in standard Italian, after a past reporting verb like ha detto, the usual way to express would in indirect speech is condizionale passato: avrebbe impastato.

Impasterebbe is the condizionale semplice, and by itself it more naturally suggests a conditional or hypothetical meaning, like she would knead it under certain circumstances. In this sentence, the idea is not really hypothetical; it is a future action seen from a past point of view.

Because impastato is part of a compound tense with avere: avrebbe impastato. With avere, the past participle usually does not agree with the subject.

So even though the subject is Marta, you still get:

• Marta avrebbe impastato

By contrast, with essere, agreement normally happens:

• Marta sarebbe tornata

That is why a feminine subject does not automatically make every participle feminine.

This is a common kind of tense shift in reported speech. A likely direct version would be:

Impasterò il pane appena torno dal mercato.

When that is reported after a past verb like ha detto, torno often shifts back to tornava.

So tornava does not necessarily mean repeated or habitual action here. It reflects the sentence from the past point of view of what Marta said.

Because Italian generally does not use the conditional like that after time words such as appena, quando, or non appena.

So after ha detto, Italian may use:

• appena tornava in a more neutral/reporting style
• or a more formal option such as appena fosse tornata

But appena sarebbe tornata is generally not the normal choice.

The subject is still Marta. Italian often leaves subject pronouns unstated when the meaning is clear from context.

So here tornava means when she came back / was coming back, with she understood automatically as Marta.

Here appena means as soon as.

So:

• appena tornava dal mercato = as soon as she came back from the market

Be careful, because appena can also mean just or barely in other contexts, but that is not the meaning here.

Literally, impastare means to knead or to make dough. In this context, impastare il pane means to make the bread dough or to knead the dough for the bread.

Italian often says il pane even when English would more naturally say the dough or simply make the bread.

Dal is the contraction of da + il.

With verbs like tornare, da can mean from:

• tornare dal mercato = to come back from the market

Compare:

• andare al mercato = to go to the market
• tornare dal mercato = to come back from the market