If it is possible for the premises of a deductive argument to be true while its conclusion is false, that argument is

If the premises can be true while the conclusion is false, the argument is invalid.

Validity means the truth of the premises must guarantee the truth of the conclusion.

Where true premises leave room for a false conclusion, that guarantee is absent.

The very possibility of this combination is the test that a valid form must pass.

Such an inference does not preserve truth, so it is invalid.