"I do not understand why."
Translation:Non capisco perché.
Unlike in Spanish, where the direction of accent marks don't really matter, Italian uses both acute AND grave accents. accents are always grave (going to the right) on a, i, o, and u; i.e. città. But e can take either a grave accent or an acute (going to the left) accent. Words ending in che take an acute accent (i.e. perché) but otherwise you use the grave accent.
They're technically pronounced differently; letters with grave accents have a wider sound than the letters with acute ones. But since e is the only vowel for which Italian uses both, it's not particularly relevant.
"Non ho capito perché" means "I have not understood why". It is the past perfect tense. http://www.edufind.com/english-grammar/past-perfect-tense/