There are some words in a Filipino sentence that does not directly carry meaning but are used as structural markers (palatandaang pangkayarian)

Structural markers:

• ang
• si
• ay
• ng
• ni
• sa

ang and si
When a noun is used as a subject of the sentence, it is always preceded by ang and si which are articles (pantukoy). Ang and si are structural markers of subjects which are always nouns.

Note that in the following sentences ang is singular and ang mga is plural:

• Ang bata ay mabait. (The child is good.)
• Ang mga bata ay naglalaro. (The children are playing.)

Note that in the following sentences si is singular and sina is plural:

• Si Sara ay kaibigan ko. (Sara is my friend.)
• Sina Paolo at Paula ay magkapatid. (Paolo and Paula are siblings.)

ay
A sentence maybe in the natural order (tuwid) or the reverse order (baligtad).
In the natural order, the subject is followed by the predicate. A structural marker ay is seen in the sentence and is an indicator that the subject comes first. In the reverse order, the predicate comes first and ay is not used in the sentence.

• Natural order: Si Mario ay katulong ng kanyang ama.. (Mario is a helper of his father.)
• Reverse order: Katulong ng kanyang ama si Mario. (Helper of his father is Mario.)

ng and ni
Ng and ni are structural markers of ownership.

• Ang pamilihan ng bayan ay malaki. (The market of the town is big.)
• Ang damit ni Sonia ay bago. (Sonia's dress is new.)

Ng can also be used as a structural marker of the object of the verb if it follows a transitive verb.

• Si Ben ay nagtanim ng palay. (Ben planted rice.)

sa
Sa is a structural marker of place.

• Si Lani ay pumasok sa opisina. (Lani went to the office.)

Reference:
Sebastian, et al. Pampaaralang Balarila ng Wikang Pambansa. 1974.

Have fun learning Filipino languages!!!