1. You use the present perfect to express what someone has done.
2. To form it you use the present of the verb haber (he, has, ha, hemos, han) and the past participle (-ar=ado and er/ir=ido)
No he reparado la bicicleta todavía
3. Remember the past participle irregulars because of the accent caer, traer, leer, oir, reir and creer and the past participles abrir, decir, escribir, morir, poner, resolver, romper, ser ver.