A triangle be congruent to a quadrilateral because congruent shapes have the same sides and angles. If the number of sides/angles is not even the same, how can the two sets be the same?
An isosceles triangle be congruent to a triangle that is not isosceles because an isosceles triangle has two sides with equal length. If a triangle is not isosceles, all its side lengths are different, so the three sides can't have the same length
congruency means that the size and shape must be exactly the same in the figures that are compared. A triangle cannot be congruent to a quadrilateral because it has three sides whereas a quadrilateral has four. An isosceles triangle cannot be congruent to a non isosceles triangle because the sides would be different lengths.
