Future Perfect Continuous Tense | Definition, Structure ...
https://www.toppr.com/guides/english/tenses/future-perfect-continuous-tenseFuture Perfect Continuous Tense lays emphasis on a duration of time before something in the future, whereas Future Continuous Tense lays emphasis on the interrupted actions or events. You cannot use the future perfect continuous tense in time clauses i.e. clauses that begin with time expressions like while, when, after, before, by the time, as long as, as soon as, if unless, etc.