Marta comprerebbe il salmone ogni settimana, purché costasse un po’ meno.

Comprerebbe is the conditional form of comprare. It shows what Marta would do under a certain condition, not what she actually does now or definitely will do.

• compra = she buys / she is buying
• comprerà = she will buy
• comprerebbe = she would buy

So the sentence is describing a hypothetical situation.

Purché means provided that, as long as, or on condition that.

It introduces the condition that must be true for the main action to happen. In this sentence, the condition is that the salmon be a little cheaper.

It is a fairly common conjunction in Italian, and it normally requires the subjunctive.

Because after purché, Italian uses the subjunctive, not the indicative or the conditional.

Here:

• costa = indicative
• costerebbe = conditional
• costasse = imperfect subjunctive

The sentence is built as a hypothetical present situation, so Italian uses:

• conditional in the main clause: comprerebbe
• imperfect subjunctive in the subordinate clause: costasse

So costerebbe would be wrong here, because the clause after purché does not take the conditional.

This sentence uses a very common Italian pattern for a present or general hypothetical idea:

• main clause: conditional
• dependent clause: imperfect subjunctive

So:

• Marta comprerebbe... = main clause in the conditional
• purché costasse... = subordinate clause in the imperfect subjunctive

This is close in meaning to the English pattern would + verb ... if + past form, as in she would buy it if it cost less.

In Italian, foods and other nouns are often used with the definite article when speaking in a general sense or about a type of thing.

So il salmone here means something like salmon as an item/product, not necessarily one specific fish already mentioned.

Italian often sounds more natural than English with the article in cases like this:

• mangio il pane
• compro il latte
• preferisco il salmone

English often drops the article where Italian keeps it.

Both can mean every week.

• ogni settimana = every week
• tutte le settimane = every week / all the weeks

In this sentence, ogni settimana is a very natural, compact way to express regular frequency.

A useful detail: after ogni, the noun is usually singular:

• ogni giorno
• ogni mese
• ogni settimana

Un po’ means a little or a bit, and meno means less.

Together, un po’ meno means a little less.

Because the verb is costare, the natural meaning in context is cost a little less, in other words be a bit cheaper.

Because po’ is a shortened form of poco.

The apostrophe shows that part of the word has been dropped. So the correct spelling is:

• un po’

Not:

• un pò

That mistaken form with an accent is very common, but standard Italian uses the apostrophe.

Yes, you could say:

Marta comprerebbe il salmone ogni settimana, se costasse un po’ meno.

That would be very natural.

The difference is mainly nuance:

• se = if
• purché = provided that / as long as

So purché emphasizes that the lower price is the condition that must be met.

Yes. The tense after purché depends on the meaning and on the rest of the sentence.

For example, with a more open present/future idea, you might see the present subjunctive:

• Marta comprerà il salmone purché costi meno.

Here, costi is present subjunctive.

In your sentence, costasse is used because the whole idea is more hypothetical: she would buy it, provided that it cost less.

The name Marta is not grammatically required if the subject is already clear from context.

Italian often leaves out subject pronouns, and sometimes even the subject noun if it is understood. So in the right context, you could simply say:

Comprerebbe il salmone ogni settimana, purché costasse un po’ meno.

But including Marta is perfectly natural if you want to identify the person clearly or make the sentence stand on its own.