How did that happen

We are told that P receives a higher grade than E and he receives a higher grade in H than in I. We know from the rules that we have a GP block. SO we know that GP > E, rule number three let's us attach H to the chain like so: GP > E > H and the question tells us that H is higher than I, so we have: GP > E> H > I. So since each of these is higher than the next, and we have 5 of them with 5 different grades, we can deduce that G will get an A, P will get a B, E will get a C, H will get D, and I will get an F. Rule number 2 tells us that His I and R grades are consecutive. Well, since I is getting an F, that means the only grade that R can get that is consecutive to the grade of an F is a D. That is why R gets a D. Hope that clears things up! Please let us know if you have any other questions.

Why is this combination not possible: G P H/E I R

This can't happen because H must get a higher grade than E, so they cannot get the same grade.