Present Perfect Tense
Present perfect tense describes an action that happened at an indefinite time in the past or that began in the past and continues in the present.This tense is formed by using has/have with the past participle of the verb. Most past participles end in -ed. Irregular verbs have special past participles that must be memorized.
Past Perfect TensePast perfect tense describes an action that took place in the past before another past action. This tense is formed by using had with the past participle of the verb.
Ex: By the time the troops arrived, the war had ended.
Ex: By the time the troops arrive, the combat group will have spent several weeks waiting.
Perfect Progressive Forms
Ex: The CEO has been considering a transfer to the state of Texas where profits would be larger.
Ex: Before the budget cuts, the students had been participating in many extracurricular activities.
Ex: By the year 2020, linguists will have been studying and defining the Indo-European language family for more than 200 years.