。
I think she knows the truth.
Literal
I [topic-は] she [subject-が] truth [object-を] knowing-[state-ている] [quotative-と] think.
Embedded clause with the ~と思う pattern. 彼女 is inside the embedded clause so it takes が (not は). 知っている uses ている for the state of knowing (not the act of learning). 真実 ('truth').