Learn Italian: Siccome c'è molto traffico, bisogna essere prudenti. - Since there is a lot of traffic, one must be cautious.

Bisogna means "it is necessary" or "one must." It is an impersonal verb, meaning it does not refer to any specific person like "I" or "you." You just use the fixed form bisogna followed directly by an infinitive verb (here, essere) to express a general necessity.

Why is prudenti plural if bisogna doesn't refer to a specific group of people?

This is a special rule in Italian! When you use an impersonal expression (like "one must be...") with the verb essere, the adjective that follows takes the masculine plural form. That is why we use the plural prudenti instead of the singular prudente, even though we aren't talking about specific people.

Can I use perché instead of siccome at the beginning of the sentence?

Generally, Italians prefer to use siccome (since or as) when starting a sentence with the cause or reason. Perché (because) is mostly used in the middle of a sentence to connect two ideas, like in: Bisogna essere prudenti perché c'è molto traffico.

Does molto change its ending in this sentence?