When I have to deal with irrealis
Irrealis is a grammatical term that covers a range of functions that are in the non-realised, imagined sphere of events, states and actions, such as future, negation, subjunctive, optative etc. It's most often described as a mood or modality, but not always. (This is related to whether future is a tense or a mood.) Typically, if the marker in question only marks future it is not called "irrealis" but "future" or "non-past" if applicable, but it might be called "irrealis" because neighbouring languages use a cognate to encode other irrealisy functions and such is the convention in that descriptive tradition (which does make sense). And of course, irrealis, future and related functions tend to grammaticalise into each other over time, and create nice fuzzy stages that we linguists can tear our hair out over (finally admitting to ourselves that while we love natural languages, they also drive us crazy). Basically, there is a lot of di...