Complex sentence: Dawn tells Tara she is glad that Tara and Willow have recovered from a major argument which she does not remember, causing Tara to suspect that Willow has used magic to alter her memory.
Split into simpler sentences: Dawn tells Tara she is glad that Tara and Willow have recovered from a major argument. Tara has no idea what Dawn is talking about, and begins to suspect that Willow has used magic to alter her memory.