A trapezoid has two sets of parallel sides.

always

sometimes

never

The answer is never. In the definition of trapezoid is states ONE pair a parallel sides, called bases.

There is some disagreement whether parallelograms, which have two pairs of parallel sides, should be regarded as trapezoids. Some define a trapezoid as a quadrilateral having only one pair of parallel sides (the exclusive definition), thereby excluding parallelograms.

In other words, sometimes.