The spatial preposition en drew study interests of some linguists and they often compared the usage of en with another preposition, dans in French. According to Anscombre (2011), phrases such as en prison, en cuisine, etc. require certain “homogeneity” between the meanings of the preposition and other elements of the sentences in question while sentences with dans are not constrained by this homogeneity. As to what these other elements are (e.g., subjects or verbs), Anscombre did not elaborate. This paper aims to clarify the constrained use of en in comparison with dans. The analysis of this paper could assist French teachers in answering some of grammar questions in French courses.