What is the difference between dhamb (sin) and sayyi'ah (evil deed)? And if dhamb is greater, how can it be forgiven while a sayyi'ah is expiated?

The verse, "Our Lord, forgive us our sins and expiate our evil deeds," means that "sins" refer to major sins and "evil deeds" refer to minor sins. Thus, forgiveness is for sins (major sins) and expiation is for evil deeds (minor sins). Expiation applies to minor sins, not major ones. Calamities expiate minor transgressions, but sins are not forgiven except through repentance or good deeds.