What is the difference between dra and gå?
"Gå" is just for walking and for certain expressions. "Dra" is the best translation for the English word "go," and "gå" the best translation for "walk."
I see "gå" used for expressing "go" and "doing" quite often; no walking invovled. In those instances, is "dra" an acceptable alternative?
No, it really depends on the context.
Jeg må dra. Folkene mine trenger meg.
If må means must and have at the same time, how should I define them
It means only one thing in Norwegian, it's just that there are several ways of expressing that one thing in English: "I must", "I have to", "I need to", "I am required to".
How would it be “The girls have to go”?
"Jentene må dra."