"An owl is not an insect."
Translation:Strigo ne estas insekto.
Esti isn't an transitive verb; it doesn't take a direct object. It is a linking verb (or a copula) and needs a subject form on both sides. English has this rule too, though we often ignore it. That's why the correct way to confirm you identify on the phone is, "this is he", not "this is him". Of course, English is changing and we no longer adhere to those rules, but at one point a mistake like that would have gotten you smacked in school ;).