bien

Bien functions primarily as an adverb modifying verbs and adjectives, meaning "well" or "properly." It also serves as a discourse filler in conversation and can introduce concessive clauses in more formal writing (bien que, though this is literary).

The common pitfall is using bien where Spanish requires bueno. Bien modifies how something is done (adverb), while bueno describes what something is (adjective). Saying "Es bien" to mean "It is good" is a widespread learner error - the correct form is "Es bueno" or "Está bien" (depending on whether you mean inherent quality or current state).