The future perfect tense is constructed by using the future form of the irregular verb haber with a past participle:
 

The past participle does not change forms when it is used in the future perfect tense.  For example, yo habré dicho, nosotros habremos dicho, vosotras habréis dicho, etc.  The future perfect in Spanish has exactly the same use as it does in English; it is used to express completion of an action in the future:  Para la fecha límite, yo ya habré terminado el trabajo  (I'll have the work done before the deadline).  The future perfect can also be used, like the future tense, to express surprise.  ¿Cómo lo habrá hecho Manuel?  (How can Manuel have done that?).