In LLPSI Cap XXXVIII Orberg scribit:
'Prius circum Siciliam tibi navigandum est.
Quod genus ‘navigandum’? Cur?
Salve pmda!
Prius circum Siciliam > tibi navigandum est> .
The expression tibi navigandum est is a passive periphrastic construction using the gerundive + esse. The gerundive (in this case that of the verb navigare) has the full set of endings for all genders. The gerundive (a verbal adjective and in this case a predicate adjective) agrees with the subject of esse, e.g. Puella laudanda est. The dative (in your sentence tibi) is called dative of agent and indicates the person (here “you”) who ought to do something (in this case navigare).
Why neuter gender? As I said, navigandum takes its gender from the subject of esse (or in this case est). But there is no “proper” subject to be found here, therefore the neuter gender (also used when the subject is an entire statement or a verb).
LatinLibrary.com has quite a few example phrases which can be found here. All these have in common that the gerundive takes the gender from a subject which in an active construction would be in the accusative, e.g.
- Passive: Senatui Romano hae litterae mittendae sunt.
- Active: [Aliquis] senatui Romano has litteras mittit.
Vale,
Carolus Raeticus
Carole, gratias tibi ago. Explanatio tua perfecta est!