
Cathy S. answered 06/03/21
Expert Mandarin Teacher
The concept you need to understand this is the "topic-comment" structure so common in Chinese. The subject is 我。The topic is the rental unit, the place being discussed. It's harder to notice that this is a "topic-comment" structure because the topic is implied/already understood.
The apartment (topic) , I stayed happily these past two days (topic).
The 的 should be 得: 住得很愉快。
Subject predicate doesn't get you anywhere in understanding the grammar of this sentence.