His math skills have regressed and now he can't even solve simple problems.

regressed 是动词 regress 的过去分词形式。regress 由前缀 re-（表示"向后、往回"）和词根 gress（来自拉丁语 gradus，表示"走、移动"）组成，字面意思是"往回走"，引申为"退步、退化"的意思。