The current paper attempts to account for the formation of English middle sentences. Discussing a set of previous analyses on the construction under investigation we show, following the assumptions of Oosten(1986) and Iwata(1999), that English middle constructions should be divided into two types: generic middle constructions and non-generic middle ones. The distinction is shown to be closely related with the tense aspect of the verbs: the structural subject in the former is interpreted as generic with the present tense, while the latter can take past tense and progressive aspect with a non-generic subject. Other thematic roles than Agent can be realized as the structural subjects. In the generic interpretation, the intrinsic property of the plays the role of cause and is primary responsible for the event denoted by middle verb. In the nongeneric interpretation, on the other hand, a specific event plays the part of cause and thus can take the past tense. Middle verbs are lexically derived from a set of activity or accomplishment verbs that carry [-state, +process] aspect features. After derivations, the verbs in the generic interpretation possess [+state, +process, +repetition] aspect feature, and those of the non-generic one, [-state, +process, -repetition]. Another contrast lies in the definiteness of the subjects: the generic interpretation involves an indefinite/generic subject and the non-generic one needs a definite/non-generic subject.