i forgot to tell you something about time and space.time and place play crucial roles in the novel.these two factors are responsible to make these boundaries shadowy because even if these lines are there they can not divide the experiences of the persons.the people on both sodes of these lines have shared experience which they can not forget easily.the riots were happened in the dhaka but roby was able to recall it after many years in an european country.thus it does not matter if these lines are there or not.this again is showed in the opinion of thmma about boundaries.there was no trenches or anything which can tell her this is india and this is dhaka.she even call this attempt useless.but then she was told that the lines are drawn on the airport itself when they check your passport etc.we can say that people dont want such boundaries but when these are drawn then they starts to think it as something real just like the make-believe story of the upside down house.then these lines become more important than the feelings of the people.
i can only give you hints..we can not sinply talk about the death of tridib.if we have to fully appreciate this episode then we also have to mention the dhaka of th'mma..
dhaka was the birthplace of thamma.she was attached to it.she was very eager to see the dhaka.but after the partition dhaka has changed completely.her journey to dhaka tells us about the shadowy boundary lines drawn between two countries.but later she did not like her beloved dhaka because it is now a different country which is full of hatred mainly because now this lines becomes firm in the conscious of the people.and this was simply the reason of the riots..dhaka episode is important in many ways because the main accident of this episode is riots resulting in the death of tridib.the jethomoshai considered dhaka his own country but it is only his daydream because the people of this very country killed him.the death of tridib forced May to think that she is responsible for his death and this death also effected Roby.
i think after this very incident thamma becomes radical nationalist.
you have to assemble all these points in your essay.in more simple terms you have to discuss the themes of nationalism time and space,sacrifice and shadow lines with reference to this dhaka episode because i think all these are combined together in this episode.