He compartmentalized the documents for different projects into separate folders.

compartmentalized 是动词 compartmentalize 的过去式形式。compartmentalize 由名词 compartment（隔间、分隔区）加上动词后缀 -ize 构成，意思是"把...分成不同的部分或类别"。compartmentalized 在这里表示"将...分类整理"的动作。