Why do we use the subjunctive with "siempre y cuando"?

Bueno, está bien. Siempre y cuando tenga una buena personalidad.

Why do we us subjunctive.

What is the rule

¡Hola Ken3883!

Thanks for your question!

We use the subjunctive here because siempre y cuando “as long as” is introducing what's called an “adverbial clause of condition.” In other words, this sentence is indicating that it is okay (está bien) only on the condition that she have a good personality (tenga una buena personalidad). Adverbial clauses of condition always take the subjunctive because the actions in them are always uncertain: they may or may not take place in the future. 

You can find a full explanation on these kinds of adverbial clauses, together with more examples, in Lesson 16.7: “When and Why With the Subjunctive.” 

I hope that this helps to clear everything up for you!

Saludos,

Liss

P.S. - I've updated the title of this thread just so that other users who have the same question can find it more easily! :)