"The dog has a long neck."
Translation:De hond heeft een lange nek.
How do I know when to use lang or lange? Is it because male and female? Do I have to remember it or is there a general rule that I can fallow?
In Dutch masculine and feminine nouns are both de woorden except for some very specific situation there is no difference. However for using lang or lange you have to know if a noun is a de woord or a het woord (neuter), so you'll have to remember that. And next to that apply a general rule that is explained here.