Learn Italian: L'articolazione rimane gonfia finché cammini. - The joint stays swollen as long as you walk.

Finché on its own means 'as long as' or 'while.' It shows that two conditions happen at the same time (the joint is swollen while you are walking). If the sentence used finché non, it would mean 'until' (it stays swollen until you walk), which would completely change the meaning!

Why does the adjective gonfia end in -a?

It has to match the noun it describes, l'articolazione (the joint). Even though articolazione ends in -e, almost all Italian nouns ending in -zione are feminine. Therefore, the adjective takes the feminine singular ending -a.

Is cammini in the indicative or subjunctive mood here?

It is in the present indicative, stating a simple fact: 'as long as you walk.' However, for regular -are verbs, the tu form is exactly the same in both the present indicative and the present subjunctive. Because this sentence describes a factual routine rather than a hypothetical situation, it takes the indicative.

Could I use sta instead of rimane to mean 'stays'?

It is much better to use rimane (from rimanere) or resta (from restare). While English uses 'stay' to mean 'remain in a certain condition,' Italian uses rimanere or restare with adjectives like gonfia. Using sta here is a common trap for English speakers and would sound unnatural.