Markers in Tagalog play a role similar to that of articles and prepositions in English.
A marker is, in fact, a word that comes before a noun.
Depending on the focus of the verb a marker indicates the role that the noun plays in the sentence: a marker may mark a noun as actor, object, location, direction etc.
ANG and SI/SINA
These markers are used to point out the focus of the sentence.
1. ANG (or ANG MGA if the focus of the sentence is a plural) marks a word as the focus of the sentence except for nouns that are the names of people that are marked by SI (singular) or SINA (plural)
Pumunta ang empleyado sa opisina (singular)=the office worker went to his office
Pumunta ang mga empleyado sa opisina
Pumunta si Mario sa opisina
Pumunta sina Mario at Grace sa opisina
NG and NI/NINA
These markers are used when the noun is not the focus of the sentence
Binasa ng estudiante ang libro=the book is what the student read (the student is not the focus of the sentence, rather it is the book)
Binasa ng mga estudiante ang libro
Binasa ni Mario ang libro
Binasa nina Mario at Grace ang libro
NG also indicates possession
Ang libro ng estudiante (the book that belongs to the student)
Ang libro ni Mario
SA and KAY/KINA
These markers have to do with direction, location and beneficiary and correspond to prepositions in English
Ibinigay ng titser ang isang libro sa estudiante (or sa mga estudiante=plural)=the teacher gave a book to the student
Ibinigay ng titser ang isang libro kay Mario (or kina Mario at Grace)
Pumupunta ako sa bahay=I am going (sa=to in this context) home
Pumupunta ako kay Mario=I am going to Mario
Mayroon ang isang Jollibee restaurant sa Milan, Italy=there is a Jollibee restaurant in Milan
SA also marks a future time
Magbabakasyon ako sa Agosto=I’ll go on vacation in August