The Markers ANG and SI/SINA

A.  ANG

The marker ANG is one of the markers used to point out the focus of the sentence.
1. ANG marks a word as the focus of the sentence except for nouns that are the names of people (like Bob, Sarah, etc.).  Below are some examples:

Abogado ang babae.	The woman is a lawyer.
Masaya ang piknik.	The picnic was fun.
Natulog ang bata sa kwarto.	The child slept in the room.
May isang kapatid ang lalaki.	The man has a brother.
Nasa mesa ang kape.	The coffee is on the table.

In each of these sentences the marker ANG indicates which word is the focus. For example, in the first sentence, babae is the focus of the sentence. We know this because it has angin front of it.
2. ANG is a noun definitizer  (i.e., it functions like the definite article the, as opposed to a or an, in English), especially in sentences using pseudo-verbs.  The following examples illustrate the contrast between definite and indefinite noun phrases.

Definite	Gusto ko ang mangga.	I like the mango.
Indefinite	Gusto ko ng mangga.	I like mangoes.
Definite	Kailangan ko ang libro.	I need the book.
Indefinite	Kailangan ko ng libro.	I need a book.

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B.  SI and SINA
The markers SI and SINA are used to indicate that the name of a person is the focus of the sentence. We already learned that ANG is used to make most words the focus of the sentence. However, ANG is not used with personal names like Bob, Mary, Jose, Rita, etc. In order to indicate that a name is the focus, the marker SI must be used.
If there is more than one name, e.g. "Phil and Tom", then the marker SINA must be used. SI comes before a single name while SINA precedes two or more names. In sentences whereSINA comes before a single name, it implies that there are other (unnamed) people who are included in a group with or are in someway associated with the person that is named. In other words,  sina Bill would mean something like "Bill and company". 

Below are some examples:

Singular Nouns
Estudyante si Rita sa UP.	Rita is a student at UP.
Mabait si Lito.	Lito is nice.
Nag-aaral si Fely sa salas.	Fely is studying in the living room.
May dalawang kotse si Dolly.	Dolly has two cars
Nasa Maynila si Lorna.	Lorna is in Manila.
Plural Nouns
Estudyante sina Rita at Fe sa UP.	Rita and Fe are students at UP.
Mabait sina Lito at kapatid niya.	Lito and his brother are nice.
Nag-aaral sina Fely.	Fely and company are studying .
May dalawang kotse sina Dolly at Joe.	Dolly and Joe have two cars.
Nasa Maynila sina Lorna.	Lorna and company are in Manila.