Genitive (Gen) is a case which marks one noun as dependent on another, for example, John's car.
In the linguistic literature, ‘realis’ and ‘irrealis’ have most often been
discussed under the more general heading of mood and modality.
Modality refers to linguistic devices that indicate the degree to which an observation is possible, probable, likely, certain, permitted, or prohibited. In English, these notions are commonly (though not exclusively) expressed by modal auxiliaries, such as can, might, should, and will.