“You taught me language, and my profit on’t
Is I know how to curse. The red plague rid you
For learning me your language!” (Act 1 scene 2 line 437-439)
This line from the play comes from Caliban speaking to Miranda and Prospero. Prospero has just accused Caliban of raping his daughter. In this line, Caliban is defending himself and expressing the anger that has been building inside of him towards Prospero for conquering the island that his mother left for him. He talks about how the only thing he gained from their teachings of their language is how to curse and he explains how he can easily use what they taught him against them. This quote by Caliban can be looked at as a rebellious and brave act, considering Prospero is his master and he can easily use magic to inflict pain on Caliban.
This line is relevant to the play as a whole because Caliban later has thoughts of revenge. This scene is just the beginning of this anger build up towards Prospero, and Caliban does soon take action. As soon as Stephano and Trinculo arrive on the island, he talks about worshipping Stephano and killing Prospero in the process. This is the first time Caliban has thoughts of having a shift of power on the island he claims belonged to him. They continue with this plan and attempt to go through with it, only to be stopped by Ariel of course. This craving of vengeance towards Prospero can relate back to Caliban’s line in the first scene where he does rebel for the first time.
I agree with your take on the quote, Caliban is clearly stating how Prospero teaching his language has helped him be able to turn it against them through cursing them. An example being Caliban telling Prospero to essentially become ridden with the “red plague” ,which is not elaborated upon but seems to be extremely unpleasant. Another thing to think about in this quote is maybe Caliban means that his only “profit” from learning their language is being able to curse them since they clearly don’t listen to what he has to say or how he is feeling. I think this would line up extremely well with the frustrations and hatred he develops for Prospero throughout the play.