当成(dàng chéng) - to regard as, to treat as in Mandarin

当成 (dàng chéng) is the verb for putting something in a category it does not necessarily belong to. Someone treats a friend as family, a listener takes a story as true, a boss treats an employee as a machine. There is nearly always a mismatch implied, which is why so many examples are complaints.

The frame is fixed: 把 X 当成 Y. The thing being reclassified moves in front with 把, and Y follows. 他把我当成傻子 is he takes me for a fool. Without 把 the sentence limps, and learners feel that immediately.

Two traps. First the tone: this 当 is dàng, fourth tone, not the dāng of 当时 or 当老师. Second, do not reach for it when you mean become. 变成 and 成为 describe a real change with no observer needed: 他变成了一个好人, he became a good man. 当成 always needs somebody doing the regarding, and the change may exist only in that person's head.