Although I am enjoying very much and benefitting greatly from Ørberg’s Roma Aeterna, I still like to dip into my old Kennedy’s Primer from time to time. I feel that I should know this construction: pefect participle in the neuter with sum. Kennedy says that intransitive verbs are used in the passive only in an impersonal construction. It seems to me a kind of periphrastic construction. Why the neuter? I would welcome any reference to other grammars where it is treated more fully.
I have looked in Woodcock “A new latin syntax” 1959 Bristol Classical Press which I find very useful.
60. The Impersonal Passive
As an active intransitive verb has no direct object to become the subject of the passive form, it follows that intransitive verbs, including those which take the dative, cannot be used ‘personally’ in the passive, i.e. they cannot have first- and second-person, or plural forms. But the third person singular passive of intransitive verbs is very common in Latin. The explanation of this form (which is, in fact, the earliest passive form) is that an intransitive verb can have a cognate or internal object (cf. Section 1), and this, whether expressed or understood, can become the subject of the ‘impersonal’ passive. Hence > curritur > means > ‘running > is taking place’, i.e. ‘people run’; > cursum est> , ‘running took place’; > currendum est> , ‘running is to take place’. > The subject, in fact, is the abstract noun, the name of the action, implied in the root of the verb. > This is true also of impersonal verbs of active form such as tonat, ‘It thunders’, pluit, ‘It rains’. > When the abstract noun implied by the verb is not expressed it is assumed to be of neuter gender, which explains the neuter form of > cursum, currendum> ; and of > pugnatum est> , ‘a battle was fought’, beside > pugna pugnata est> . > As with the personal passive, an agent with the impersonal passive (except for the gerundive, see Section 202) is expressed by ab with the ablative.
My underlining.
Thank you, Seneca. That is most helpful.