@2nailsScrumpy
The answers given to you are correct. This is just to explain further why your response was corrected by your Filipino friends during dinner:
When you use an object-focus verb like “kinain”, the object has to be specific and is assumed to be in its entirety.
We use the “ANG (the)” before the object of an object-focus verb because it is THE specific object. That explains the correct translation given by Beemo of your response – I already ate THE dinner. Since the persons with you asked you why you are not eating your DINNER with them, that dinner becomes the specific object based on context when you responded with "kinain" as your verb. So, with your reply to them, it would not really make sense for you to have eaten that dinner (and in its entirety) because it is what they were still having at that moment.
When you use a subject-focus verb like “kumain”, the object is nonspecific and is assumed to be just a portion of whatever that object is. With a subject-focus verb, the object will be preceded by “NG” instead of “ang”. That “ng” is used only to indicate what the nonspecific object is - “Kumain na ako NG hapunan” = I already ate dinner. It can be any dinner and obviously not the one they are having at that time.
You can use any of the following as the proper response to their question:
Kum
ain na ako. = I already ate.
Nakak
ain na ako. = I have already eaten.
Kumain na kasi ako (ng hap
unan) (kanina). = Because I already ate (dinner) (earlier). - as suggested by PinoyTaj.
Nakakain na kasi ako (ng hapunan) (kanina). = Because I have already eaten (dinner) (earlier).
The last 2 sentences would sound quite formal though.
Here’s another example to further explain the difference between the objects of subject-focus and object-focus verbs.
Let’s say that there are 3 apples and 2 oranges in a bag:
“KUMAIN ako NG mans
anas na nasa bag” will be understood as that you only ate one or two of them or any fraction of them, e.g., 1/2 of one, 1 plus 1/3 of the second one, 2 plus 1/4 of the third one, etc., as long as you did not eat all of them. The oranges remain untouched
“KINAIN ko ANG (mga) mansanas na nasa bag” will be understood as that you ate all three of them. You may even omit the “mga” because the object of “kinain” will already be understood as the entire group of apples in the bag. The oranges remain untouched.
“KUMAIN ako NG prutas na nasa bag”. = I ate some of the fruits in the bag. Something still remains untouched in the bag - it may be just a portion of the apples or just a portion of the oranges or portions of both apples and oranges.
“KINAIN ko ANG (mga) prutas na nasa bag” means you ate all five of them.