I would like to know the difference between "1" and "2".
1. I took you for ( =believed that you were)an old friend.
I have taken his statement for granted. (= believed his stament to be absolutely true)
2. I mistook you for(= confused you with) an old friend. I often mistake my girl friend for her mother on the phone.(=confused her with her mother)
Please note that mistook for is phrasal verb having construction = mistake sb/sth for sb/sth.