hello guys. in a few days i have an exam. i have two problems to solve, but i couldn't do it. so please help me

1) show that "a" is false in a given interpretation if and only if "-a" (negation of a) is true in the same interpretation. and "a" is true if and only if "-a" false.

2) show that none of the formula of First order can't be false and true at the same time.

please guys help me. and if u can please keep it as simple and as "short" as it can be.