Question: I am an American, I live in the Philippines. At a rece

Because your reply translates to "I already ate the dinner", which doesn't make sense in this case. 'Kinain' from the verb 'kainin', is an object focus verb. So your reply focuses on what is being eaten rather than who does the eating. Which is why 'kumain' - an actor focus verb- makes sense here. Bc kumain focuses on who does the eating rather than what is being eaten.

Also, and this is just a guess, I try and go with very simple stuff, the simpler the better I think to get the point across, than add words that make it perfect later, at least that is my approach to language learning. I used to focus so much on grammar and this is a poor method since you can't really translate each word from English to Tagalog or Spanish as it just doesn't work and confuses me more. I go with bulky chunks now, for example, I would try to say, "food, I like". or "go, you there". I hope that makes sense.

Kumain na kasi ako (ng hapunan) (kanina).