Learn Italian: Se fosse più forte, non avrebbe perso la partita. - If she were stronger, she wouldn't have lost the match.

This is a mixed conditional. It describes an ongoing, present condition (she is not a strong person in general) that had a consequence in the past (the match is already over). If the condition were also entirely in the past ("If she had been stronger at that moment"), you would use the pluperfect subjunctive: se fosse stata più forte.

How do we know this sentence is about "she" rather than "he"?

Out of context, we actually don't! Both fosse and avrebbe are third-person singular verbs, which can mean "he", "she", or the formal "you" (Lei). The adjective forte doesn't give it away either. The translation says "she" based on the wider context of the conversation.

Since the subject is female, why isn't the adjective "forta"?

Forte belongs to a group of Italian adjectives that end in -e in the singular. These adjectives are identical for both masculine and feminine subjects. They only change for number, becoming forti in the plural.

Is "perso" the only past participle for "perdere"? I've seen "perduto" before.

Both perso and perduto are correct past participles of perdere. However, perso is much more common in modern, everyday spoken Italian. You will usually see perduto in literature, older texts, or poetic set phrases like "tempo perduto" (lost time).