Any verb can be substantivised (is that a word?) very easily, i.e. by adding the affix -οнт and a noun/adjective suffix.
E.g.: ειδι ("he sees") > ειδοнтιc ("he who sees / the see-er"); гυнαc δεδαcεc ("women will (hopefully) have given") > δεδαcοнтιαc гυнαc ("the women who (hopefully) will have given").
Also, infinitives can be turned into nouns (and adjectives) simply by adding a neuter suffix. Here, the nominative and accusative are the same as the infinitive:
Participles are used in many more cases that most modern natlangs. For instance, participles can be used as any wordgroup in a sentence, i.e. subject, object etc.
Also, participle adjectives can be used to give extra information about any wordgroup in a sentence.