坚 is a bound morpheme (b.f.) in Mandarin, meaning it cannot stand alone as an independent word. It usually appears as the first character in two-character compounds that express physical hardness (坚固, 坚实), abstract firmness or determination (坚持, 坚强, 坚定), or military fortifications (堡垒 is more common; 坚 alone is rare). In surnames, it is pronounced Jiān. Learners should focus on the compound patterns rather than treating 坚 as a free-standing adjective or noun.