Learn Italian: A vedere il palazzo fatiscente, si scappa. - If you see the dilapidated building, you run away.

This sentence uses an implicit conditional clause. By combining the preposition a with an infinitive verb like vedere, you create a condition. It translates closely to 'Upon seeing' or 'If one sees'. This is a more advanced, natural alternative to using se (if) with a conjugated verb like se si vede.

Does the person seeing the building have to be the same person running away?

Yes! When you use the a + infinitive structure, the implied subject of that action must match the subject of the main clause. Because the main clause here uses the generic si scappa (one runs away), the seeing is also done by that generic 'one'. If the sentence ended with scappo (I run away), the condition would automatically become 'If I see...'.

Who is actually doing the running away in 'si scappa'?

The phrase si scappa uses the impersonal si construction. This means the subject isn't a specific person, but rather 'one' or 'people in general'. While it literally means 'one escapes', conversational English usually expresses this generic, universal reaction using 'you run away'.

Is fatiscente just another way to say 'ugly'?

Not exactly. Fatiscente is a very specific B2-level adjective that means 'dilapidated', 'crumbling', or 'falling apart'. It is almost exclusively used to describe physical structures like buildings (il palazzo) that are decaying or in a ruinous condition.