Question

Is a trapezoid always a polyhedron

Answer 1

No, a trapezoid is at least one pair of parallel sides. 
A Polyhedron is a 3 dimensional shape with flat faces - Not all are parallel, therefore it is not always a trapezoid. :-)

Answer 2

 well you probably already know a convex quadrilateral with at least one pair of parallel sides is referred to as a trapezoid

Then:

so before hand of knowing the sytem which i mean is it always

of course not a trapezoid could never become an "Always" in the form polyhedron

Hope this helped ;)