An argument is invalid if and only if (points : 1)

An argument is invalid if and only if all the premises is true and the conclusion is false.

We can test the invalidity of the argument by assuming all the premises are true and seeing whether it is possible for the conclusion to be false. If the conclusion is possible to be false, then, the argument is invalid.

A valid argument, on the other hand, is valid if and only if it is necessary that:
1) if all of the premises are true, then the conclusion is true
2) if all of the premises are true, then the conclusion must be true
3) it is impossible that all the premises are true and the conclusion is false.