Question: "ng" and "sa"

I'm not really sure there is a hard rule here, I think sometimes they are interchangeable, but in this case inyo is a SA pronoun so it requires sa.Perhaps: Ang lihim na iyán ang pápatáy ng relasyón mo , would also be valid.I will await the feedback of a native speaker!

@AMBoy But they also say (1) instead of (2), i.e. they use "kay" instead of "ni". (1) Sino ang pumatáy kay JFK?
(2) Sino ang pumatáy ni JFK?

@kasong Kay = Sa , you use Kay when Its a name , Kay is for names and sometimes Filipinos will personify nouns. This was explained to you on your previous post .

@kasong (2) Sino ang pumatáy ni JFKThis is wrong and does not make sense. Ni marks ownership.Kay can also be used to denote ownership but can also denote action done to the object. Ni can't be used in this case

Ang form Si
Ng form Ni
Sa form Kay Si John
Ni John
Kay John@kasong As a non - native this is how I’ve learned to work it out and make it make sense.

Here is the real answer and I learned something. This makes sense. Smart Native:
Ah, the actual reason why "sa" is used here is because of definitiveness. "Relasyon" here, whether it be "inyong relasyon" or "relasyon mo," is specific; there is a specific relationship being discussed here. As such, when an agent-focused verb is used, it prefers to take "sa.". In the first sentence, the "lamok" that the child killed is just any mosquito; it doesn't pertain to any specific mosquito, so it takes "ng."My thoughts, this makes sense:Sino ang pumatáy kay JFK? (JFK is definite)
Sino ang pumatáy ng mga tao? (People is indefinite) And yeah ni is wrong.

You mean, the sentence (a) below is possible, if the noun "lamok" is definite??(a) Pumatay sa lamok ang bata.

@AMBoy You mean, the sentence (a) below is possible, if the noun "lamok" is definite??(a) Pumatay sa lamok ang bata.

@kasong Sa is used for direction.The correct marker here ng

@Bituingmaykinang Yes, OK. But can you explain why the direction marker "sa" is used in the following sentence? Ang lihim na iyán ang pápatáy sa inyóng relasyón.We use also the direction marker "kay" in the following sentence, which I cannot understand. Sino ang pumatáy kay J. F. Kennedy?The two example sentences above have common that the "ang" marker comes before the verb.