Getting through passive periphrastic is proving difficult.
Are we talking about the future passive periphrastic of indirect speech?
And with the addition of dative of agent I'm confused. So every time I want to express an action that has to be done I use dative of agent? i.e. Future passive participle + Esse + Dative.
Ah! You seem to be on about the gerundive. Another issue entirely.
Pons est aedificandus
Here, Pons changes from being the object to the subject. What happens if we want to say who built the bridge?
Civibus patria defendenda est
Patria remains in the nominative, and the gerundive still agrees with it. But the agent is in the dative. If you like you can think of this as a dative of interest (so far as the citizens are concerned...). This dative needn't be mentioned, as we saw above.
If there is another dative in the sentence, confusion can sometimes arise. If we say consulibus tribunis parendum est
or pecunia mihi vobis reddenda est
, then who gives money to who, or who obeys who? This is an instance where we'd use the ablative of agent with a gerundive: consulibus a tribunis parendum est
or pecunia a vobis mihi reddenda est
. This is done only
if confusion would arise. Laws can't obey people, so omnibus civibus legibus parendum est
has only one meaning and is the correct form.
There are of course other ways to express obligation or necessity.
Debeo aut fustem aut lapidem capere
Aut fustem aut lapidem capere me oportet
In some circumstances a jussive subjunctive can be used: caperes aut fustem aut lapidem
: you ought to have taken a stick or a rock.
Since we're dealing with passives of intransitive verbs that take dative objects, I'll add what I've learned about the subject here.
Normally intransitive verbs (ones that don't take objects, like to go, to come, to fall) can't be made passive in English. No-one could say "It was gone to London by me yesterday". But this is sometimes possible in Latin.
Itur in antiquam silvam
: It is gone into an ancient wood, or they go into an ancient wood.
Likewise: Ab hostibus acriter pugnatum est
: It was fought fiercely by the enemy, or the enemy fought fiercely.
This is the only passive construction usable by verbs in the dative: omnibus argumentis utendum est
: use must be made of all arguments.
the consuls are obeyed.
There, bit of a braindump, but I think everything you want to know should be there...
If you want to know about the future passive periphrastic in indirect speech, that's for another time.