He ____ every letter his grandmother wrote to him during the war.

Erklärung

During the war places the whole habit in finished time, so the past simple is needed, and keep shortens its vowel and ends in -t to give kept. Because this verb has only two written shapes in total, kept also appears after have, as in he has kept them ever since. Keeped and kepted both bolt a regular ending onto a past that is already irregular.