Hi Adam.

I gotta say no, it is not necessary to use "di" since the sentences are not a passive form.

Both of those can be used and are understandable.

This is something he likes

- Ini sesuatu yang disukainya (literally: This is something liked by him)

- Ini sesuatu yang dia sukai

This is something he bought

- Ini sesuatu yang dibelinya (literally: This is something bought by him)

- Ini sesuatu yang dia beli

Terima kasih, semoga membantu :-)

I hope this is help you.

1st person singular = I = Aku  , prefix"ku"

1st person plural = We = Kami, Kita there is no prefix for kami and kita

example :

- this is our book, Jane. Ini buku (milik) kita, Jane.

2nd person singular = You, prefix "mu"

2nd person plural = Kalian , and there is no prefix for "Kalian"

3rd person singular = He/She = Dia , prefix "nya"

3rd person plural = They = Mereka ,  i think you should use "mereka"

example :

This book belongs them

Buku ini milik mereka