The nominative subject of the third person may be omitted
a. When it is expressed or implied in the context:
ὁ σὸς πατὴρ φοβεῖται μὴ τὰ ἔσχατα πάθῃ
b. When the subject is indefinite, especially when it is the same person or thing as the omitted subject of a preceding infinitive ( cross937 a): ἡ τοῦ οἴεσθαι εἰδένάι (ἀμαθία_), ἃ οὐκ οἶδεν
c. When a particular person is meant, who is easily understood from the situation: τοὺς νόμους ἀναγνώσεται
d. When it is a general idea of person, and usually in the third person plural of
e. In descriptions of locality: ἦν δὲ κρημνῶδες
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].