cum eas longi erroris taederet

Latin Learning Latin
1

In LLPSI Orberg scripsit:

Inter eos ludos feminae Troianae, cum eas longi erroris taederet, naves incenderunt..

Now my understanding is that that which is being tired is in accusative and that which is the thing of which one is tired is in genitive. But what determines the number of taederet? Why doesn’t this sentence read:

'…cum eas longi erroris taederent ?

2

Impersonal verbs are always in the third person singular. There’s no subject for the verb to agree with in person or number. Think of the subject as something like English “it.”

3

Many thanks