Before I proceed to explain the difference between “niya” and “kaniya”, I feel obliged to introduce to you the 2 kinds of simple sentence construction in Filipino (since this is important):
1. Karaniwang Pangungusap (Regular Sentence). This is easily distinguished by the presence of the linking verb "ay". "Ay" is the Filipino equivalent of the verb "to be" (is/am/are).
Example:
A. Siya ay maganda.
B. Ikaw ay matalino.
C. Ang telepono ay sinagot niya.
2. Di-karaniwang Pangungusap (Irregular Sentence). Without the presence of "ay".
A. Maganda siya.
B. Matalino ka.
C. Sinagot niya ang telepono.
I introduced this to you because many Filipinos have forgotten about this. Since Irregular Sentences are used more often in daily and informal conversations, many Filipinos can't recognize the true subject of the sentence anymore.
Take our third example:
"Hindi niya sinagot ang telepono." (This is actually in the passive voice)
This can be translated as "He/she didn't answer the phone." (If you ask any Filipino, he/she will automatically answer you with this.)
But literally, preserving the grammatical structure of the sentence, it means "The telephone was not answered by him/her." (passive voice)
Many Filipinos will fail to recognize that in this sentence, the subject is "Ang telepono", and not "niya" which they would then mistakenly equate with "siya" (subjective pronoun). You will know this if you convert the sentence to the Karaniwang format, "Ang telepono ay hindi niya sinagot."
“She didn’t answer the phone” (active voice) literally translates to “Siya ay hindi sumagot sa telepono.” Or “Hindi siya sumagot sa telepono.” “Siya” is the subject.
This misunderstanding always happens when the "Di-karaniwang Pangungusap" (Irregular) sentence is used because the predicate comes first.