My grandfather recorded his experiences during the war in a series of letters.

recorded 是动词 record 的过去式形式。record 由前缀 re-（表示"再次"）和词根 cord（源自拉丁语 cor，意为"心"）组成，原意是"再次记在心里"，引申为"记录、记载"的意思。recorded 在这里表示"记录下来的"动作。