Circumstances and actions can be expressed in different ways depending on the time they took / take / will take place.

In English, 'time' (Past, Present or Future) is expressed by categories, called 'Tenses', such as e.g. Simple Present tense and Present Progressive tense for the time we call "Present":

Tim goes to school every day. (Simple Present Tense)
Tim is writing a letter. (Present Progressive Tense)


A tense is formed by special forms of verbs, the past tense e.g. is formed by the past tense form (also called 2nd form) of the verb; the will-future tense is formed by will+infinitive of the verb:

(I)write - (II)wrote - (III)written = Tim 'wrote' a letter yesterday.


The past participle (also called 3rd form) is another form of the verb which is used to form e.g. the passive voice:

(I)write - (II)wrote - (III)written = The letter has been 'written',
(I)go - (II)went - (III)gone = He will have 'gone' tomorrow;

or the "Perfect Tenses" like e.g. the Present Perfect:

He has 'gone' to school.
He has 'written' a letter.