In The God of Small Things, pages 209-214, 275-278, 314, and 321, how did forbidden love play a role in the novel?

I don't know if the copy I have of this novel corresponds to yours in terms of page numbers. I recommend that you visit the pages suggested by your teacher in the question to double-check which specific elements are being referred. These are fairly narrow page brackets which should guide you towards your answers.
Forbidden love, however, certainly plays a significant role in this novel—indeed, it is the absolute core theme. Forbidden love is the driving force behind the experiences of Rahel and Estha and those around them, whose world is governed by the divisions enforced by the caste system and the "Love Laws," which must not be transgressed. All the real romantic relationships in the novel—those between Ammu and Vilutha, Baby Kochamma and Father Mulligan, and the quasi-romantic incestuous love of Rahel and Estha—transgress the laws of caste and society. At the conclusion of the novel, we see the twins, who have been so destroyed by this system in which love must obey rigid rules, falling back upon the most forbidden love of all: their connection with each other.
You identify Chapter 11 as one of the sections for discussion in terms of how it contributes to this theme. In Chapter 11, the "one-armed man" of Ammu's dream is described as a representation of "The God of Loss" or "The God of Small Things." He is defined by his inability to "win" his love: "If he touched her, he couldn't talk to her, if he loved her he couldn't leave...if he fought he couldn't win." This is a highly symbolic passage in the novel—Ammu has been "happy" with her dream man, but the children question whether such happiness "counts" if it isn't real and cannot endure. It is suggested that Ammu has only ever achieved such happiness in her own imagination. The closing passage in the novel explains how "the small things" between Ammu and her forbidden lover, Velutha, were those things to which they confined themselves in discussion to allow "biology" to take over, and to prevent having to consider the "big things" which divided them, and which must inevitably tear them apart.
After Ammu wakes from her dream, she and her twins listen to a song from a film whose story is again of forbidden love: "a poor girl who is forced to marry a fisherman from a neighboring beach, though she loves someone else." At the end of the story, "everybody dies... The sea claims them all." This story represents in microcosm what is suffered by all the lovers in the novel: where love is real, it ends sadly, and where society attempts to put constraints upon love, "everybody," metaphorically, "dies." Ammu's relationship with Velutha, the Untouchable, cannot progress without tragedy—"a small price to pay"—and the song to which Ammu and her twins dance in this chapter foreshadows the inevitability of this fate.
At the end of the novel, after Estha and Rahel reunite, they are described as "Quietness and Emptiness fitted together like stacked spoons." Although they are together, and can understand each other as nobody else can, they share "not happiness, but hideous grief," knowing that "once again they broke the Love Laws. That lay down who should be loved. And how. And how much." The only true love in this novel is forbidden love; relationships that do fall within social constraints, such as Ammu's with her husband, are unhappy, but equally, those who transgress the Love Laws are punished, and know that this is the reason why.