it is better to seek forgiveness than to ask permission → κρεῖττόν που συγγνώμην ὕστερον ζητοῦν ἢ ἐξουσίαν πρότερον αἰτεῖν

[cyclops92]

A translation of this would be very apreciated. So far, I have found this: Είναι καλύτερα να επιδιώξει τη συγχώρεση από το να ζητήσει άδεια but I'm not sure if this is modern or ancient greek and I'm also not sure if it is correct.

[crystal]

First of all, I think you already know that the original quotation is: "It's easier to ask for forgiveness than it is to get permission"

The famous quotation "It's easier to ask for forgiveness than it is to get permission" is often attributed to Grace Hopper.
Grace Hopper - Wikipedia

which in Greek would be translated as:

Είναι πιο εύκολο να ζητήσεις συγχώρεση για κάτι που έκανες παρά άδεια για να το κάνεις.
http://www.thegreekz.com/forum/archive/index.php/t-166262.html

Then, JFK altered it a bit and gave us the quotation you asked for:

John F Kennedy

It's better to seek forgiveness than ask permission

which in Greek would be translated the same but substituting εύκολο with καλύτερο or προτιμότερο:

Είναι καλύτερο/προτιμότερο να ζητάς συγχώρεση για κάτι που έκανες παρά την άδεια για να το κάνεις.

[mavrodon]

The translation is in modern Greek. Wait for Bill's response.

[crystal]

Ναι, όντως, συγγνώμη για τα νέα Ελληνικά. It should be in ancient Greek.

[billberg23]

So now, Cyclops, you know how it should look in beautiful modern Greek.  Ordinarily, we recommend that, in the case of a quotation like this that was originally composed and well put in a modern language, you stay with a modern language rather than forcing it pointlessly into a dead language.  But if you really need to know how the phrase (as you wrote it) would have looked in ancient Greek, it would be something like
κρεῖττόν που συγγνώμην ὕστερον ζητοῦν ἢ ἐξουσίαν πρότερον αἰτεῖν.