Why писать becomes пишу
Separate a stem alternation from the personal ending.
Писать has infinitive/past stem писа- but present stem пиш-. The present paradigm is built from пиш-, so пишу is learned as a principal form rather than guessed as *писаю.
- Stem change: писа- → пиш-.
- Ending choice: я uses -у after ш.
- Learn non-transparent verbs with key finite forms.
Two layers in пишу
First identify the stored present stem, then apply spelling and ending information.
писать → present stem пиш-пиш- + -у → пишуMini-check