Complementary infinitive

A complementary infinitive is a verb in the infinitive form which is used with an indicative verb to complete the idea or action of the verb.

The complementary infinitive can be present or aorist:
 * present - to express am action viewed as a process or ongoing
 * aorist - to express an action viewed as a complete whole

Examples

 * present example: Οὐ δύναμαι ἐγὼ ποιεῖν ἀ ἐμαυτοῦ οὐδέν (I am able to do nothing on my own, John 8:30) - δύναμαι is the indicative verb "able" and ποιεῖν is the present infinitive "to do" which completes the idea of "able".
 * aorist example: δυνήσεται ἡμᾶς χωρίσαι ἀπὸ τῆς ἀγάπης τοῦ θεοῦ (will be able to separate us from the love of God, Romans 8:39) - χωρίσαι is aorist infinitive.

Observations
It seems the choice of present or aorist infinitive is a matter of subtle nuance. In the case of "able to do", the speaker is telling the listener, see, all the things I am doing.... So he is looking at the things he is doing as onging processes. He is doing, and will continue to be doing these things. In the aorist example, "nothing will be able to separate us", the speaker is not interested in elaborating whether the act of separation is a process or an event since it isn't going to happen. He wants to emphasise that "nothing can". So he uses the aorist.