τῷ δὲ νόμῳ πειστέον καὶ ἀπολογητέον.
The law must be obeyed and a defence must be made.
The verbal adjective in -τέον (rarely -τέᾰ) serves as predicate NP with ἐστί (rarely εἰσί). The object is in the case required by the verb (accusative, genitive or dative) and has the macrorole of patient.
The acting role may be expressed by a dative of the agent. Sometimes the acting entity is put in the accusative, as if it were the subject of an infinitive clause with δεῖ. In the latter case ἐστί is always omitted.
The impersonal construction with -τέον is much more common than the personal construction with -τέος, -ᾱ, -ον. In Sophocles 85% of cases are impersonal (Moorhouse apud Duhoux).
πῶς οὐκ τὴν εἰρήνην ποιητέον ἡμῖν;
Why should it not be our duty to secure peace?
καὶ τῆς κενώσεως ἄρα φροντιστέον αὐτῆς μόνης, ἀμελητέον δὲ τῆς γενέσεως;
Are we to pay attention merely to the decline of this honour, and not to its origin?
σαφηνείας γὰρ ἕνεκα καὶ τούτοις τοῖς ὀνόμασι χρηστέον
For the sake of clarity, therefore, these names must also be used.