"I am their son."

Translation: मैं उनका बेटा हूँ।

[KaterinaGricenko]

Why can't it be "unke"? Why "unka"?

[Jeevikaa_nan]

Beta is singular masculine noun
So we use "unka" beta.

Possessive pronouns in English are my , your,our, his, her,its, their.

मैं # मेरा /मेरी/मेरे
तू # तेरा/ तेरी / तेरे
तुम # तुम्हारा/ तुम्हारी /तुम्हारे
आप # आपका/ आपकी/ आपके
हम#हमारा/हमारी/हमारे

यह#इसका/इसकी/इसके
वह#उसका/उसकी/उसके
ये#इनका/इनकी/इनके
वे#उनका/उनकी/उनके

1st Person Singular Masculine Possessive Pronoun is मेरा and 1st Person Plural Masculine Possessive Pronoun is मेरे which are different but 1st Person Singular Feminine Possessive Pronoun is मेरी and 1st Person Plural Feminine Pronoun is also मेरी.(Its common in all respective persons.)

मेरा दोस्त = My Friend.
मेरी घड़ी = My Watch.
मेरे बच्चे = My Children.
मेरी घड़ियाँ = My Watches.

तेरा कुत्ता = Your Dog.
तेरी दोस्ती = Your Frienship.
तेरे कान = Your Ears.
तेरी सब्जियाँ = Your Vegetables.

तुम्हारा पलंग = Your Bed.
तुम्हारी बहन = Your Sister.
तुम्हारे खिलौने = Your Toys.
तुम्हारी गाड़ियाँ = Your Cars.

आपका प्यार = Your Love.
आपकी चूड़ी = Your Bangle.
आपके पंखे = Your Fans.
आपकी चाबियाँ = Your Keys.

हमारा प्रयास = Our Hardwork.
हमारी कुर्सी = Our Chair.
हमारे गधे = Our Donkeys.
हमारी गलतियाँ = Our Mistakes.

इसका जेब = His/Her/Its Pocket.
इसकी माँ = His/Her/Its Mother.
इसके अंडे = its eggs.
इसकी तितलियाँ = His/Her/its Butterflies.

उसका हाथ = His/Her/Its Hand.
उसकी बीवी = His Wife.
उसके कुत्ते = His/Her/its Dogs.
उसकी उंगलियाँ = His/Her/its Fingers.

इनका हीरा = Their Daimond.
इनकी तस्वीर = Their Photo.
इनके ताश= Their Cards.
इनकी गलियाँ= Their Street.

उनका बेटा = Their Son.
उनकी बेटी = Their Daughter.
उनके बेटे = Their Sons.
उनकी बेटियाँ = Their Daughters.

[BoogalooBa]

Thanks

[vinay92]

You use उनका, उनके or उनकी depending on whether the noun that is its object is singular(male), plural(male) or female respectively. SInce बेटा is singular, you should stick to उनका.
Compare with the sentences
हम उनके बेटे हैं - We are their sons
मैं उनकी बेटी हूँ- I am their daughter
हम उनकी बेटियां हैं- We are their daughters

[neil_05]

What is the difference between उसका and उनका?

[vinay92]

उसका is 'his/her' while उनका is 'their'.

However, उनका may also be used to refer to a single person when you are being respectful/formal.

[Path966366]

When do you use आपना आपने आपनी???

[vinay92]

अपना/अपनी/अपने (note that the first character is अ not आ) is used when you want to refer back to the subject of the sentence or clause.

For example, in the sentence 'I am going to my brother's house', the subject is 'I'. The pronoun 'my' is referring to the same noun as the subject of the sentence. In such a situation, Hindi would use आपने as the pronoun (instead of the usual pronoun for 'my' which is मेरे). The translation would be 'मैं अपने भाई के घर जा रहा हूँ'.

Similarly, in 'Julia is playing with her cat' if the pronoun 'her' is referring to 'Julia' who is the subject of the sentence, it would be translated as अपनी. This sentence would be translated as 'जूलिया अपनी बिल्ली के साथ खेल रही है'.

Similar to other possessive pronouns, the choice of अपना, अपनी or अपने is made depending on the gender and number of the noun that comes after it.