sino ang ...?

Due to Tagalog's predicate-initial order, the interrogative pronoun sino almost always sits at the very beginning of the sentence. It is strictly an ang-case pronoun used primarily for humans, though it is also frequently used for pets treated as family members. When followed by the ang marker, it creates a structure to identify a specific individual or group. Enclitic particles like ba, po, or na typically follow sino directly, before the ang marker (e.g., Sino po ang guro?).

This pattern attaches to various word types:

• Common Nouns: Sino ang guro? (Who is the teacher?)
• Verbs (Nominalized): When a verb follows ang, it functions like a noun. The verb can be in any aspect. The verb's focus must match the role of the person being identified. Use actor focus (like -um-) if the person performed the action: Sino ang kumatok? (Who knocked?) or Sino ang kumakatok? (Who is knocking?). Use object focus if the person is the receiver: Sino ang tatawagin mo? (Who will you call?).
• Adjectives: Sino ang maganda? (Who is the beautiful one?)
• Plurals: The standard way to ask about multiple people is to use the reduplicated form Sino-sino (often spelled Sinu-sino in older texts). However, in casual speech, Filipinos often use the singular Sino with the plural marker mga: Sino ang mga kasama mo? (Who are your companions?)

Proper Names and Pronouns:
When asking about specific names or using pronouns, the ang marker is replaced or dropped:

• Proper Names: Replace ang with the personal marker si for one person, or sina for multiple people: Sino si Maria? (Who is Maria?), Sino sina Juan at Pedro? (Who are Juan and Pedro?).
• Pronouns: Drop the marker entirely when using personal or demonstrative pronouns: Sino ka? (Who are you?), Sino ito? (Who is this?).

Important Conversational Notes:

• The 'Sinong' Contraction: Sino ang frequently contracts to Sinong (formally spelled Sino'ng to indicate the dropped 'a' in 'ang') before verbs and adjectives (e.g., Sino'ng kumatok?). Be careful not to say Sinong ang, which is redundant. Also note: when Sinong is used directly before a noun, it translates to "Which" rather than "Who is the" (e.g., Sinong guro? means "Which teacher?", whereas Sino ang guro? means "Who is the teacher?").
• Using 'yung: In casual speech, the marker ang is often replaced by 'yung (e.g., Sino 'yung kumatok?).

Note that sino is strictly an ang-case pronoun for people (and beloved pets). For objects or general animals, use ano ang. Additionally, do not confuse sino with its other case forms: nino (ng-case, used for possession or non-focused actors) or kanino (sa-case, whose / to whom). Sino cannot be used to show possession. For example, "Whose is this?" is Kanino ito?, whereas Sino ito? means "Who is this?".