びる / びて / びた

びる turns a noun into a verb meaning "to look like", "to have the air of": the thing has really taken on that quality. It goes straight after the noun, or after the stem of an い-adjective with the い dropped, and conjugates as a る-verb. You meet it mostly as びた before a noun.

The words it makes are few, and each is in the dictionary as a word of its own: おとなびる大人びる "look grown-up", ふるびる古びる "look old and worn", ひなびる "look rustic", いなかびる田舎びる "look countrified". You cannot add びる to any noun you like. Outside 大人びる, which you also hear in conversation, they belong to written description of places and things.

びる or ぶる

ぶる is putting on an act; びる is real. A child who is おとなぶっている大人ぶっている is pretending to be grown-up, and a child who is おとなびている大人びている really does seem older than they are.

Describing a state

At the end of a sentence, a look that something already has takes 〜びている, and a change happening now takes 〜びてきた. A bare 〜びる to end a sentence is rare.