“Dati” and “ná
una” are both used to refer to things that happened or existed in the past. However, they can’t be used interchangeably.
“Dati” is often used as an adverb (e.g. formerly, previously, once, then, before), but may also be an adjective (former, previous), and may also translate to the verb “used to (+ infinitive)” at times. It refers to an unspecified time in the past.
una” is a verb. It is the past tense of “má
una” – NÁ
. The root word is “una (first)” and “mauna” means “to be first/ahead (of a set, which could also be within a set)”. For example, if the 2nd, 3rd, and 5th place winners in a track race are Andrew, Charles, and James, respectively, then it is correct to say any of the following:
Nauna si Andrew kay Charles. = Andrew was ahead of Charles.
Nauna si Charles kay James. = Charles was ahead of James.
Nauna si Andrew kay James. = Andrew was ahead of James.
Nauna si Andrew kina Charles at James/James at Charles. - The order of the names of Charles and James would not matter because “nauna” is all about Andrew.
So, Andrew, Charles, and James belong to a set/subset within the larger set of all those who competed in the race.
DATI si Jennifer Aniston ang asawa ni Brad Pitt. = FORMERLY/PREVIOUSLY/IN THE PAST/ONCE Jennifer Aniston was Brad Pitt’s wife.
Magkaklase kami DATI. = We were classmates THEN/BEFORE. = We USED TO BE classmates.
Siya ang DATI naming boss. = He is/was our FORMER/PREVIOUS boss.
NAUNA si Jennifer Aniston kay Angelina Jolie na maging asawa ni Brad Pitt. = (Lit: Jennifer Aniston was ahead of Angelinia Jolie to become Brad Pitt’s wife.) Brad Pitt was married to Jennifer Aniston before Angelina Jolie.
Hindi kami magkaklase kasi NAUNA siya sa akin ng isang taon. = We were not classmates because he was AHEAD of me by a year.
Ilan ang NAUNA ninyong boss BAGO yung ngayon? = How many bosses CAME/have you had BEFORE your current boss? - “Bago” normally means “new” but when used with “nauna” or in the context of a series, it translates to “before” – “nauna bago” = came before.
Your sentence “I saw the PREVIOUS president earlier today.” = Nakita ko ang DATIng (dati na) pangulo kanina.
But I also saw the president BEFORE him there. = Pero nakita ko rin ang NAUNAng (nauna na) pangulo sa kaniya doon.
“Previous” is also “bago” when used in the context of a series of events.
Previous to this ... = Bago ito ...
Previous to her being the vice president. = Bago siya naging vice president.
We sometimes use “nakaraá
n (was able to pass by/came to pass)” to mean “previously”, especially as a prelude to the continuation of a TV series. We’d say “ang nakaraan” to mean “previously on ...”.