Is it Possible to have a situation when E-U < 0? Explain

Dear student,
The kinetic energy is always positive because it is one half the mass times the speed squared.
$K.E=\frac{1}{2}m{v}^2$
The mass is always positive and the square of the number is always positive and hence kinetic energy is positive.
Potential energy can either be positive and negative For example,the electrostatic potential energy of proton and electron separated by distance r is 
$P.E=-\frac{k}{r}$
where k is positive number.So the potential energy is negative.If the magnitude of potential energy is greater than kinetic energy,then the total energy is negative .

Regards.