hi newbie here. i have been getting used to and playing around with the basics and have been trimming surface in various ways but unless theres 4 edges i cant seem to repair them.

ie i took a U shaped trim from the edge of a surface and cant seem to repair the U shaped bite. whats the answer when theres less than 4 edges ?

do i need to repair the edge first ?

no doubt theres lots of ways to do this but baffled right now