There are three types of ‘be+to-infinitive’ in English: ‘be to,’ ‘BE TO,’ and ‘is to.’ The differences between these types are fully discussed in this study. They have an apparently identical construction in which the verb ‘be’ is followed by an infinitive, but in fact they have different syntactic and semantic characteristics. Jespersen(1940) classified English infinitives into three ranks: primary, secondary and tertiary. The verb ‘be’ of ‘be to’ is a copula and the infinitive is Jespersen’s primary and so a nominal infinitive clause acting as complement of ‘be.’ There are two kinds of infinitives as Jespersen’s secondary: retroactive and non-retroactive. Korean school grammar has argued and taught that Jespersen’s non-retroactive infinitive of BE TO is a subject complement, and so an adjective equivalent. However, this argument has proved to be untenable. Unlike ‘be to,’ ‘BE TO’ cannot be divided into two units, i.e. the verb ‘BE’ and ‘TO-infinitive.’ The whole ‘BE TO’ as a unit is a modal idiom or quasi-modal referring to the future. The infinitive of ‘is to’ is a retroactive. It is an active infinitive which is said to have a passive meaning, but Jespersen(1940) looks upon the infinitive as active and as governing a preceding item as its object.