Emitting smoke and flames but not fully burning

धधकता हुआ

English Usage: The smoldering ruins of the house were a grim reminder of the fire.

Hindi Usage: घर के धधकते हुए खंडहर एक भयानक यादगार थे।

To burn slowly without flames

धीरे-धीरे जलना

English Usage: The coals continued to smolder long after the fire had died down.

Hindi Usage: आग बुझ जाने के बाद भी कोयले धीरे-धीरे जलते रहे।