YOU SAID:
He is sitting next to me and working on the computer
INTO JAPANESE
彼は私の隣に座ってコンピューターで仕事をしています
BACK INTO ENGLISH
he is sitting next to me working on his computer
INTO JAPANESE
彼は私の隣に座ってコンピューターを使っています
BACK INTO ENGLISH
he is sitting next to me and using the computer
INTO JAPANESE
彼は私の隣に座ってコンピューターを使っています
BACK INTO ENGLISH
he is sitting next to me and using the computer
Well done, yes, well done!