Why Use "ΝΤ" Instead Of "Δ"

Καλησπερα - I have a bit of a beginner question here - a pattern I see is that in some Greek words, when making the "d" sound, instead of using the Delta letter, the language uses ΝΤ to make the "d" sound - Could someone please explain why this is, and if there is a certain rule in place where you know whether to use Δ or ΝΤ?

Ευχαριστώ πολύ