Learn Italian: Fra tutte le città, Roma è la più bella che abbia visitato. - Of all the cities, Rome is the most beautiful that I have visited.

The subjunctive is triggered by la più bella, which is a relative superlative (saying something is the most or the least of a group). When a relative clause starting with che describes a superlative, Italian uses the subjunctive because it frames the statement as a subjective, personal experience rather than an absolute, universal fact.

How do we know abbia visitato means 'I have visited' and not 'he/she has visited'?

The present subjunctive auxiliary abbia is the same for io, tu, and lui/lei. Usually, context makes it obvious. Since this sentence expresses a personal opinion about travel, io is the most natural default. If you needed to clear up confusion, you could add the pronoun and say che io abbia visitato.

Why is the past subjunctive (abbia visitato) used when the main verb (è) is in the present tense?

The tenses reflect when each action happens. Rome is currently beautiful (present tense è), but the visits took place in the past. Abbia visitato is the past subjunctive, formed with the present subjunctive of avere plus the past participle, which perfectly matches the English 'have visited'.

Could I use tra instead of fra at the beginning of the sentence?