。
She knows that John loves her.
Literal
She [topic-は] John [subject-が] loves-for-her [thing-of-の] [object-を] knows.
Both verbs are in the present continuous: the love (愛してくれている) and the knowing (知っている) coexist in time. Note that 知る in Japanese requires ている for the resultant state of having come to know — bare 知る would refer only to the moment of acquiring the knowledge, not the ongoing state of possessing it. The nominalizer の embeds the clause '[John is loving her]' as the object of the knowing, where と would have made it a quoted thought. の-nominalization is the standard pattern with verbs of perception or awareness — 見る, 聞く, 知る, 感じる — distinct from こと-nominalization, which prefers more abstract or factual contexts.