Learn Italian: Marco non ha ancora visitato la mostra. - Marco has not yet visited the exhibition.

In Italian, many adverbs of time like ancora (yet), già (already), and mai (never) go right in the middle of a past tense verb. They sit between the helping verb (ha) and the main action verb (visitato).

Does ancora mean 'yet', 'still', or 'again'?

It can mean all of those depending on the context! However, when you pair it with non (not) to make the phrase non ... ancora, it specifically translates to 'not yet'.

If the sentence was about Marta instead of Marco, would visitato change to visitata?

No, it would stay exactly the same: Marta non ha ancora visitato. In this past tense (the passato prossimo), verbs that use avere (ha) as their helper verb always keep their default -o ending, no matter who is doing the action.

Is the noun mostra (exhibition) related to the verb mostrare (to show)?

Yes, exactly! Mostrare is the verb 'to show', and la mostra is a noun meaning 'the exhibition' or 'the display'. Connecting nouns to their related verbs is a great way to build your Italian vocabulary.