geani, nu prea te-ai impacat cu gramatica.
"i did not understand" -> "understand" e aici la modul infinitiv (mod nepredicativ). "understand" e un verb neregulat, adica nu-i adaugi "ed" ca sa formezi trecutul.
"understand/understood/understood" -> cele 3 forme (infinitiv, simple past, past participle)
had (not) + past participle = Past Perfect
have (not) + past participle = Present Perfect
http://www.englishpage.com/irregularverbs/irregularverbs.html