-il-

-il- names the instrument that performs the root's action, so the root is nearly always a verb. tranĉi gives tranĉilo, ŝlosi gives ŝlosilo, skribi gives skribilo, veturi gives veturilo, and flugi gives flugilo, a wing, which shows that the tool need not be manufactured. The result is a noun in -o, and ilo also stands alone meaning tool.

The competition is with -ist- and -ej-: the same verb gives you the doer, the place and the implement, so kuiristo, kuirejo and kuirilo divide the kitchen neatly between them. Beware the ordinary roots that end the same way. facila, simila, trankvila and utila are adjectives with no suffix inside them.