This is a compound tense, so two verbs are required: the main verb and the past participle of whatever verb you're using. The past participle does not have to agree in gender or number with the subject. The formula is:

HABER + PAST PARTICIPLE OF VERB

Some examples:

1. I would have entered if you had opened the door. = Yo habría entrado si hubieses abierto la puerta.
2. I wish you had spoken. = Ojalá que hubieses hablado.

Irregular past participles:

Some past participles are irregular in all the perfect tenses.

1. For -er and -ir verbs, if the stem ends in a vowel, a written accent will be required.
2. Some verbs are irregular in the past participle form, so these verbs are considered irregular in the perfect tense. Here are the most common irregular past participles: