~kinda late answer~
There is no rule, it's really very complicated even for us greeks. In general, just remember that you use "δεν" when the word which follows starts with a vowel.
The correct grammar demands to use "δεν" also when the next word starts with "κ, π, τ, γκ, μπ, ντ, τσ, τζ, ξ, ψ" but mainly it's for helping with the pronunciation like "a" and "an" in English.
But there is a lot more, you just scratched the surface. My advice is to use it as suits you because no one is going to pay attention and correct you.
English isn't my native language so sorry for any misconception.