"Ako" is used as a SUBJECT / OBJECT
"Ko" is used as POSSESSIVE PRONOUN
Example:
Ako ay matangkad. (I am tall.)
Ako ay naglalakad. (I am walking.)
Tinawag mo ako. (You called me.)
Ito ang pagkain ko. (This is my food.)
"Ko" an also be used as a contracted form of "Ako" (as OBJECT)
Example:
Tinawag mo ko. (You called me.)