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An example of past, present, future tenses?

let's say "KAIN" for "EAT"

Here's how you it goes with the tenses (past, present, future):

KUMAIN / KUMAKAIN / KAKAIN > for simple S-V or S-V-O or S-V-Adv. sentence pattern

kumain siya (he has eaten) / kumain sya ng mabilis (he ate hurriedly)
kumakain siya (he is eating) / "
kakain sya (he is going to eat)/ "

KINAIN / KINAKAIN / KAKAININ > these are used for S-V-O sentence pattern

kinain nya ang mansanas (he ate the apple)
kinakain nya ang mansanas (he is eating the apple)
kakainin nya ang mansanas (he's going to eat the apple)

... these verbs type can also be used with inversion (where predicate comes before the subject):

well, Tagalog is pretty confusing at the start... you'll get used to it with a lot of practice.

good luck.

:)