"Effing" is used as an intensifier or an adjective and is a milder replacement for a more offensive word', "f******g". "Dammit" is used as an exclamation and is a contraction of "damn it". For example, "Dammit, I locked my effing keys in the car!"

Dammit is an exclamation. Like if you hurt yourself you would say "Dammit!" Effing is usually used as an adjective. You have to incorporate it into a sentence. Example: "This line is too effing long!"