"They read that book."
Translation:वे वह किताब पढ़ते हैं।
It is a good question, because it actually could have been पढटी as well; It is because the sentance was "they read THAT book" and not "they read THIS book";
So between the two following sentence you had to choose the first one : Ve VOHA kitab parte hain Ve YEH kitab parti hain व वह कीटाब पढते हैं व ये कीटाब पढटी हैं
So पढती could have been correct too, since "ve" can refer to women as well
"वे वह किताब पढ़ते हैं।" sounds weird.
'They read that book.' sounds better translation for "वे उस किताब को पढ़ते हैं।". The reason is: the book is being referred or pointed to by the demonstrative that/उस and thus it becomes the definite direct object of the verb 'read'/पढ़ना, and takes the postposition को .
यह and वह both are stand alone third person singular pronouns. यह, meaning he/she/ it, is used for s.o./sth. close and वह with the same meaning is used for s.o./sth. distant or absent.
Hindi translation for 'That book is read by them.', which is in passive voice, is वह किताब उनके द्वारा पढ़ी जाती है।.