The idiom "心不在焉" (xīn bù zài yān) is formed through a classical Chinese grammatical structure where "心" (heart/mind) serves as the subject, "不" is the negation "not", "在" means "to be at/in", and "焉" is a classical function word meaning "here" or "in this place". Literally translating to "the heart/mind is not here", it functions as a complete metaphorical sentence describing a state of mental absence or distraction. Its formation follows a simple Subject + Negation + Verb + Location pattern, which over time solidified into a fixed four-character idiom used to critique someone's lack of attention or preoccupation.