English verbs, except Modal, have three forms. The forms help express thought about Time (PRESENT, PAST or FUTURE) and Aspect (Simple, Progressive, Perfect, or Perfect Progressive).
The first form can make the Present Simple:
The second form can make the Past Simple:
The third form can make the Perfect:
I have written (Present);
I had written (Past);
I will have written (Future).
The free PDF here has the story: ■→Appendix 2, Irregular Verbs, Low and High Vowels.