a/the key

'The' sounds better, but 'a' can be used as well.

Hard work is only one of the keys to success. There are other keys out there.

Either would be acceptable.

1. Hard work is a key to success.--suggests that there is more than one key to success, among which is hard work.

2. Hard work is the key to success.--suggests that only hard work is the key to success.

The second form--Hard work is the key to success--might be said to be preferable, because one assumes that there is always only one "key" to any "lock," in the sense of the pattern of the key. However, #1 as i have listed them is not incorrect.

Thjanks, guys, for your replies.