Is there a way to know when a verb ending in -nout will have the -nul ending or another. ex: odpočinout si = odpočinul si zapomenout = zapomeněl Is there some explanation to help or we must simply memorize the endings.
It is quite simple: 1. if the root ends in a vocal, the past participle ends in -nul: mi-nouti: minul (root mi-) to-nouti: tonul (root to-) 2. if the root ends in r, the past participle ends in -nul: hr-nouti: hrnul (root hr-) tr-nouti: trnul (root tr-) 2. if the root consists merely of ONE consonant, the past participle ends in -nul: h-nouti: hnul (root h-) l-nouti: lnul (root l-) u-s-nouti: usnul (root -s-) 4. other verbs in -nouti have usually the past participle without the ending -nul: tisknouti: tiskl padnouti: padl dotknouti: dotkl kopnouti: kopl klesnouti: klesl etc. The forms tisknul, padnul, dotknul, kopnul, klesnul, ... are considered non-standard.
The verb zapomenouti has the past participle zapomenul (the root ends in vocal: zapome-). The form zapomněl is from another verb: zapomníti, most forms of this verb are archaic, but the past participle zapomněl is still living and succesfully competes with the form zapomenul.