Learn Italian: Firmiamo il contratto, affinché l'accordo sia valido. - We sign the contract, so that the agreement is valid.

Sia is the present subjunctive of essere. We have to use the subjunctive here because of affinché (so that). Conjunctions of purpose like affinché always require the subjunctive mood because they express a goal or intention, not a stated fact.

Can I use perché instead of affinché here?

Yes! In everyday spoken Italian, people usually use perché to mean 'so that.' Affinché means the exact same thing but is a bit more formal. Just remember that when perché means 'so that' (rather than 'because'), it also requires the subjunctive: ...perché l'accordo sia valido.

What happens if the subjects of both halves of the sentence are the same?

That is an important distinction! We use affinché plus the subjunctive when the subjects of the two actions are different, like in this sentence where 'we' sign and 'the agreement' is valid. If the subjects are the same (for example, 'We sign the contract so that we are safe'), you would drop the conjunction and just use per plus the infinitive: Firmiamo il contratto per essere al sicuro.

How would this sentence change if we signed the contract in the past?