An Overview of the Present Perfect:

The present perfect is used to describe an action completed in the past, but that still has some pertinence for the present, e.g.: He decidido de venir contigo. It has two parts: a) an auxiliary form of haber and b) a past participle, e.g.: hablado. Unlike the imperfect, which is used to describe situations in the past, and the preterite, which describes relatively succinct events that have taken place, the present perfect is used to describe relatively succinct events that have taken place AND that have relevance to the present, e.g.: ¿Has terminado el libro? He terminado el libro, por eso voy a la biblioteca. 

How to form the Present Perfect

The present perfect is formed by using the present tense of haber, followed by the past participle. This is illustrated in the following table:

How to form the Past Participle?

Most past participles are formed by dropping the infinitive's ending and adding -ado for -ar verbs (e.g.: comprado) and -ido for -ir and -er verbs (e.g.: venido, sabido). That said, a number of common verbs do have irregular past partciples. Some of the most common ones are:

ser: he sido

poner: he puesto

romper: he roto

hacer: he hecho

decir: he dicho

estar: he estado

volver: he vuelto

Past participle agreement

In Spanish, there is never gender or number agreement between a past participle and a noun when the auxiliary is haber, e.g.: La chica que he encontrado. The verbal ending following haber is always -ado. However, the past participle can also function as an adjective or follow ser/estar, in which cases there is gender and number agreement with the preceding noun e.g.:

• Una chica confundida.
• Las tareas están terminadas.