Learn Italian: Quella clausola è valida, a patto che Lei rispetti la scadenza. - That clause is valid, on the condition that you respect the deadline.

Because it follows the conditional conjunction a patto che (on the condition that). In Italian, conjunctions that establish a condition or a requirement for the main action to happen always trigger the subjunctive.

Why does rispetti end in -i if the subject is the formal Lei?

In the present indicative, an -are verb like rispettare would end in -a for Lei (Lei rispetta). However, in the present subjunctive, all singular forms (io, tu, lui/lei, and formal Lei) of -are verbs end in -i. So the formal subjunctive form here is indeed rispetti.

Why is Lei capitalized in the middle of the sentence?

Capitalizing Lei is the standard way to show you are using the formal 'you' rather than the literal pronoun for 'she'. While you will sometimes see it lowercase in casual writing, capitalizing it is highly recommended in formal or business contexts, such as when discussing a clausola (clause).

Could I use purché instead of a patto che in this sentence?