-ej-

-ej- names the place where the root happens. lerni gives lernejo, loĝi gives loĝejo, kuiri gives kuirejo, vendi gives vendejo. The result is a noun in -o, and the root is usually a verb, though a noun root works too: preĝejo is built on preĝi, while kafejo sits on kafo.

The nuance worth holding on to is that -ej- gives you a room, a building or a site, never a container. A place you put things in is -uj- or -ing-, so a shoe shop is a ŝuvendejo but a shoe box is not. And do not stretch it into abstraction: an office as an institution or a post is a different idea from the room it occupies, and Esperanto keeps those apart.