ケンは、わたしとあなたを間違えた。

tendon

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

I; me; private affairs; personal matter; secrecy; selfishness

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

you (referring to someone of equal or lower status); dear (what a wife calls a husband)

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to make a mistake (in); to commit an error (e.g. in calculation); to confuse; to mistake something for something else