You're right about it being action based. The Ang pronouns (ako, ikaw, tayo, etc.) are actor focus, used for who is doing the action. The Ng set (ko, mo, natin, etc.) are object focus, or what the action is happening to.
The pronoun you choose is largely based on which conjugation of the verb you are using. You choose the conjugation based on what meaning you are hoping to convey. Conjugations are also classified as actor, object focus, etc.
e.g.
Kumain KA na ba? = "Did you eat already?" -um- and ka are actor focus.
Kinain MO na ba? = "Did you already eat it?" The "it" is implied because the -in- and mo are object focus, meaning that the thing which was eaten ("it") is the main actor in the sentence.
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