When writing congruent quadrilaterals, the order of writing the letters is very important. This is because, each side/angle in the first quadrilateral is congruent to its corresponding side/angle in the second.
Now, we are given that:
ABCD is congruent to EFGH, the letters corresponsponding to EF in the other quadrilateral is AB.
Therefore,
EF is congruent to AB